Christopher N. Warren is project manager and co-founder (with Daniel Shore) of the
Daniel Shore is Associate Professor in the Department of English at Georgetown University. Shore’s research and teaching are on the literature of the Renaissance, with a special focus on the works of John Milton. His publications include
Jessica Otis (@jotis13) is a CLIR-DLF Postdoctoral Fellow in Early Modern Data Curation at Carnegie Mellon University. She received her MS in Mathematics and PhD in History from the University of Virginia. Her research focuses on the ways people in early modern Britain used numbers and mathematics in their daily lives. For more information, see www.jessicaotis.com.
Lawrence Wang is pursuing his PhD degree in Statistics at Carnegie Mellon University. His main research area is in statistical methods for inference on network data. He has also done work in extracting relational data from text.
Mike Finegold is Vice President - Analytics at Fulcrum Analytics and a Visiting Research Scientist at the Heinz College at Carnegie Mellon University. He has held faculty positions with the statistics department at Carnegie Mellon University and the school of information systems at Singapore Management University, where his research focused on modeling consumer preferences, inferring latent network structures, and designing marketing experiments for social networks. Prior to academia he worked for several years as a management consultant, business development manager, and educator.
He received an AB in mathematics from Princeton University and a PhD in statistics from the University of Chicago.
Cosma Shalizi is an associate professor of statistics at Carnegie Mellon University, and an external faculty member at the Santa Fe Institute. He got his Ph.D. in theoretical physics from the University of Wisconsin-Madison in 2001. Website: http://www.stat.cmu.edu/~cshalizi/.
This is the source
In this paper we present a statistical method for inferring historical social networks from biographical documents as well as the scholarly aims for doing so. Existing scholarship on historical social networks is scattered across an unmanageable number of disparate books and articles. A researcher interested in how persons were connected to one another in our field of study, early modern Britain (c. 1500-1700), has no global, unified resource to which to turn. Manually building such a network is infeasible, since it would need to represent thousands of nodes and tens of millions of potential edges just to include the relations among the most prominent persons of the period. Our
We describe in this paper the natural language processing tools and statistical graph learning techniques that we used to extract names and infer relations from the
The authors present a statistical method for inferring historical social networks from biographical documents.
Historians and critics have long studied the ways that early modern writers and thinkers associated with each other and participated in various kinds of formal and informal groups. Although their findings have been published in countless books and articles, there is currently no way to obtain a unified view of the early modern social network. A scholar must start largely from scratch if she seeks to understand complex relations between multiple people, identify potentially important relationships that have yet to be explored, understand the extent of communities of interaction, or visualize the scholarly consensus regarding networks, whether small or large. The creation of a large scale early modern social network gives scholars a visual way to explore scholarly knowledge of relationships and to see what has – or hasn’t – been studied in the extant historiography.
The most desirable outcome of our work would of course be a comprehensive map of the way early modern persons were related. Yet practical challenges abound. The population of Britain rose to over 5.5 million people by the end of the seventeenth century, and little documentary evidence survives on much of that population. Attempting to reconstruct the full network would be unrealistic. Even if we limited ourselves to people alive in 1700 and successfully gathered 5.5 million names, the number of potential relationships in that set exceeds 15 billion. Social relations are exceedingly complex, even in societies considerably smaller than our own. There are thus excellent reasons to proceed more conservatively–focusing only on small, well-documented subsets of the population. Some of the best known digital networks projects, such as Stanford University’s
Taking a different approach, we identified biographical data as the most productive starting point for our network reconstruction, which we have named
the whole of our cultural inheritance has to be recurated and reedited in digital forms and institutional structures
from bibliographical to digital machines.SDFB tackles a related but more difficult problem: the
From the ODNB biographies of persons who lived between 1500-1700 we created an initial dataset of 13,309 actor nodes. Each actor node could potentially be connected to any of the other nodes, leading to over 88 million potential edges to explore. Even within this initial dataset, already limited for manageability, it was not feasible to verify each potential edge. One approach might have been to curate these relationships in an ad-hoc order, as a scholar became interested in a particular relationship or as relationships were explicitly documented in a scholarly source. We would then have collected as many relationships as the time and labor of scholars allowed, but we would have had little to say about the relative importance of collected relationships and nothing at all to say about those relationships yet to be curated. For instance, would the absence of an edge mean that the two nodes shared no association or that the association has yet to be explored in our network? Rather than rebuilding the network by hand, we chose to employ a computational and statistical approach, unifying the dispersed knowledge already extant in the literature into an inferred graph of the network that can then be made available to scholars for correction and curation.
In the following sections, we lay out our statistical method for reconstructing the early modern social network in four broad steps, then examine the significance and limitations of our results from the perspective of humanist scholarship. In section one, we discuss the process of identifying a collection of textual documents to use as input, considering both direct and indirect evidence of historical relationships. In section two, we explain how we used Named-Entity Recognition (NER) to process the unstructured text into structured data – specifically a matrix of documents and named entities – that was amenable to statistical analysis. In section three, we give an overview of how we applied statistical graph-learning methods to our structured data, with more detailed technical information included an appendix. In section four, we discuss methods of validating a sample of proposed relationships using the local expertise of humanist scholars. In section five, we step back to examine the broader significance of this process from the perspective of twenty-first-century researchers in the humanities. We also examine the assumptions underlying our statistical methods and potential areas of modification necessary before redeploying these methods with other historical corpora.
In developing this method, we have demonstrated the feasibility of applying graph learning methods to any large collection of biographical text – early modern or otherwise. This is neither a completely automated process nor a perfect one, but we have also developed a practical mechanism by which expert feedback can improve the network as well as the statistical procedures used to infer it. We have thus created a viable and transferrable approach to inferring large-scale historical social networks, which should be of particular interest to digital humanists, scholars of networks and prosopography, as well as scholars interested in the history of scholarship itself.
The first step of our process was identifying the extent of available texts and determining which texts were potentially the most useful for network inference. Numerous types of primary and secondary sources can provide evidence of historical relationships. Some of these sources provide direct evidence of a link between two actors – for example, society membership rolls, marriage certificates, or archival letters. Other sources may collectively provide indirect evidence: the same two people mentioned together in numerous accounts or biographies is highly suggestive of the possibility that those two people may have come into contact with one another.
In an ideal world we would have made use of all the relevant historical sources and scholarship. In this one, we needed to begin with a collection of texts that was well-defined, accessible, machine readable, and relatively uniform. We also wanted to begin with a collection that included a broad range of potentially relevant figures, according to social, geographic, and temporal standards. We therefore decided to focus on the 58,625 biographical entries that make up the ODNB. Running to sixty volumes in its print format, the ODNB is the labor of 10,000 scholars who have collectively contributed its 62 million words.
On a technical level, the ODNB was praised upon its 2004 release for being the first to exploit the
potential of electronic publication on so vast and imaginative a
scale
many possibilities opened up by
the online version for accessing and organising the hoard of
information
Bacon’s life and career
during the 1590s was dominated by his close relationship with Robert
Devereux
– as well as numerous implicit indicators of potential
relationships. Robert Cecil, for example, is mentioned five times in one section
of Bacon’s biography
As we worked with the ODNB data, a further advantage of this particular
collection emerged: its ability to shed light on the current state and history
of scholarship. Individually, each document in the ODNB is a roughly
chronological account of one person’s life, specifically an individual deemed by
nineteenth-, twentieth-, or twenty-first-century editors to have in some way influenced [British]
national life
In our era of text mining and network visualizations, such biases have continued
effects. A bias towards men is a known issue in existing historiography; this
bias is neither confined to the ODNB nor particularly surprising. However,
transforming textual secondary sources into visual representations allows for
more purposeful critical scrutiny of what is
known, how, and by whom
– the branch of knowledge increasingly
referred to as metaknowledge
After having identified our collection of source materials, we then had to
process the unstructured text – specifically a collection of HTML-formatted
documents acquired through the ODNB website – into a format more amenable for
analysis. This was done by extracting only the biographical portions of the text
from the initial HTML documents – stripping the HTML formatting, bibliographies,
and other extraneous text from the documents.
These NER tools use probabilistic methods to identify names and to classify
those names according to types such as person, location, or organization. For
example, the following sentence – The occasion
of ‘Lycidas’ was the death of Edward King, a fellow of Christ's College who
had drowned off the coast of Anglesey on 10 August 1637
– might be
processed as The occasion of
‘[PERSON]Lycidas[/PERSON]’ was the death of [PERSON]Edward
King[/PERSON], a fellow of [ORGANIZATION]Christ's College[/ORGANIZATION]
who had drowned off the coast of [PLACE]Anglesey[/PLACE] on [DATE]10
August 1637[/DATE]
Christ’s Collegewhere the the first word separated from its follower could mistakenly if understandably be classed as a person.
For both Stanford and Lingpipe, we began with the default models trained on news article corpora and ran the tools on ten randomly chosen documents from the ODNB. These documents were then manually tagged to determine the accuracy of the tools’ performance on our target dataset. Two measures of accuracy were used: recall, the fraction of desired results obtained, and precision, or the fraction of obtained results that are correct. For our purposes, high recall was considered necessary, while high precision was desirable but less important. Stanford achieved better recall than LingPipe, at 70.7% and 67.8% respectively, but combining their results led to recall rates of 85.7%. The two tools were combined by taking all of Stanford’s smatches, and then adding in LingPipe’s matches if Stanford did not tag those specific words. In case of overlapping or contradictory tags, we used Stanford’s matches.
Recall and Precision for Various Subsets of NER ResultsWe then implemented two additional procedures to improve recall and precision.
First, to improve recall, we ran the documents through NER twice: once to create
the initial tags and a second time using the initial tags as a dictionary, which
enabled us to search for missed instances of phrases that were tagged during the
first pass through the documents. This latter search was particularly successful
at capturing partial name co-references, which occur within documents when
historical figures are referred to only by their first or last name. With few
exceptions, partial names that are part of a longer name found in that document
are not actually different people. Bacon
in a document containing
Francis Bacon
will refer, except in rare cases, to Francis Bacon. If
a partial name matched the subject of a biography, it was considered a mention
of that subject. Otherwise, partial names were considered mentions of the
matching most recent full-name mention.
Second, to improve precision, we implemented manual rules to reduce the number of
non-human names detected. This included removing all phrases that contained
words beginning with lower-case letters; exceptions were made for the words
of
and de
which often form part of names during this period,
i.e. Katherine of Aragon.
We also eliminated phrases with non-alphabetic
characters – such as $, *, and numbers – and common non-human proper names
supplied by our subject matter experts – such as Commonwealth,
Catholic,
Greek,
and Roman.
This resulted in final recall rates of 96.7% and precision rates of 65.5% on the initial test set. Testing on six new randomly-chosen documents led to a similar 95.3% recall rate but a slightly lower 54.0% precision rate. As our priority was a high recall rate, this was deemed acceptable. A later examination of a random 200-entity sample indicated the overall dataset’s precision rates were approximately 59% with +/- 7% margin of error.
From these results, we created a large table of documents and named entities.
For each document, we tabulated the named entities and their number of mentions,
which led to 494,536 different named entities occurring throughout the
collection of 58,625 documents. We then reduced the number of named entities in
two ways. First, we ignored named entities that did not occur in an ODNB
biography within the period of interest (1500-1700). This made network inference
less costly computationally. So too with our second step, in which we omitted
names that occurred in fewer than five documents. Since correlations are very
difficult to determine with sparse data, inferring relations among low count
documents would have increased the number of false positives. While the five
mention threshold did unfortunately mean that we had to eliminate many less
prominent individuals, or those referred to by different names across the ODNB,
the tradeoff was that it helped us achieve better precision at less
computational cost. A final stage required further human curation –
specifically, searching for names in the ODNB – to disambiguate people who
shared the same name and de-duplicate people referenced under multiple names,
particularly for names obtained through the NER tools. While recent research in
the NLP community has focused on finding a way to automate this final stage,
such as the Berkeley Entity Resolution System, we preferred the accuracy of
manual curation
The resulting table of 58,625 rows and 13,309 columns is known mathematically as
a matrix. This ij, a
non-negative integer for each document/person pair. We used this document-count
matrix to infer the social network.
We motivated our statistical model for the previously described document-count
matrix by assuming that direct connections between historical figures would be
reflected by their being mentioned together in documents. Indeed, prior work has
shown it possible to infer a rough graph based on co-mentions alone
Consider an example such as the one displayed in Figure 2. George Villiers, Duke of Buckingham (1592-1628), knew King Charles I (1600-1649), and Charles I knew Prince Rupert of the Rhine (1619-1682), but Buckingham and Prince Rupert – whose lives only barely overlapped – never met. Because Prince Rupert and Charles I are connected, they will tend to be mentioned together in source documents. How often Prince Rupert is mentioned can therefore be predicted in part from how often Charles I is mentioned. Likewise if Charles I and Buckingham are connected, mentions of Buckingham predict mentions of Charles I. But in the case of no direct tie between Prince Rupert and Buckingham, as here, their names may still correlate due to mentions of Charles I. Despite such correlation, mentions of Buckingham convey no information about mentions of Prince Rupert not already accounted for by mentions of Charles I. We thus reasoned that co-mentions found in our document-count matrix – and correlations between any two given nodes derived from the matrix – might be the result of one or more confounding factors.
Under these assumptions, inferring the existence of network connections is the
same problem as inferring the conditional independence structure in a particular
statistical model – in this case, our document-count matrix p×p correlation matrix θ̂, where two
nodes j and k are conditionally
independent if and only if the coefficient θ̂ jk =
In some applications of graphical models to infer network structure, all non-zero
coefficients are of interest. For example, in gene networks, the expression
levels of two connected genes may be negatively (conditionally) correlated. In
our social network, however, we are primarily concerned with positive
coefficients, as a relationship between two people should lead to a positive
conditional correlation of their mentions in a document. A small or zero
correlation suggests a lack of relationship between two people, while negative
correlations might occur for a variety of reasons, including non-overlapping
lifespans or two-degree – i.e., friend-of-a-friend – relationships without
so-called triadic closure
We therefore used our initial correlation matrix to create an adjacency matrix Y
– a symmetric p×p matrix where
Yij=Yji=1 when there is
a positive correlation and assumed relationship between person i and person j, and 0 otherwise. Because our
data and methods provide more information about some edges than others, however,
we wanted to be able to attach a confidence estimate to potential edges instead
of simply obtaining a yes or no estimate.
Confidence estimates were also better suited to the grey areas of humanistic
research often requiring interpretation and even guesswork. In order to create
this confidence estimate, we fit the Poisson Graphical Lasso on random subsets
of our data 100 times and added the resulting adjacency matrices into a final
matrix that we called our confidence matrix, C. This calculation gave us a
confidence level
for the likelihood of a relationship’s existence
that ranged between 0 – never inferred – and 100 – always inferred.
Throughout this process, we experimented with tuning parameters and found that our final estimates did not vary significantly for all reasonable tuning parameters, where reasonable is defined as a low enough penalty such that edges are actually added, but high enough penalty that the algorithm converges rapidly. We also conducted penalty parameter training – using expert knowledge to manually confirm the existence of some relationships – but found this produced only very localized changes and had little impact on the overall network structure. The only significant manual intervention in this basic method thus came from our name disambiguation procedures, as we had nearly one thousand non-unique names in our node set. To deal with the the challenge of multiple individuals sharing the same name, we first disallowed positive adjacency estimates between two people with non-overlapping lifespans (with a one-year margin of error for posthumous children). Second, we used probabilities based on biography length to distribute adjacency estimates among people with overlapping lifespans.
A fuller explanation of our application of the Poisson Graphical Lasso can be found in our Appendix, along with a link to our code.
Having constructed our confidence matrix of estimated relationships, we then conducted three different types of validation checks: one to ensure that our results showed the homophily that network studies have taught us to expect when semantic context is taken into account; one to confirm that our results were consistent with statistical theory; and one to evaluate the accuracy of our results in comparison with an expert human reading of the ODNB biographies. We first used topic modeling on approximately 90% of our dataset – excluding people with duplicate names whose relationships had to be disambiguated – to evaluate different kinds of actor connectivity in a semantic context. Then, on smaller subsets of our data, we compared our results with alternative statistical methods, and calculated precision and recall rates.
Our first validation step was motivated by the fact that the Poisson Graphical
Lasso counts names but ignores semantic context. As a way to test the validity
of this approach, we wanted to compare the connectivity of actors who are
mentioned in similar contexts to the connectivity of those mentioned in
different contexts, since actors who share contexts are more likely to know one
another than those who do not
To generate our topic model, we created a ‘bag of words’ for each person in our
dataset, comprised of all words that appear before and after the person’s name
in the ODNB. Specifically, for each person in the network, we located all
mentions in the ODNB, and used the previous fifteen words and next twenty-five
words – excluding named entities – as their bag of words
. The choice of
these two numbers was motivated by attempting to capture the current sentence
and the previous and next sentences. We then removed all named-entity mentions
in these biographies and converted the remaining words into lower case. Next we
applied the Porter stemmer historical significance
as given by the ODNB.
Using each of these bags of words
as an individual text, we fit three
topic models to our collection of texts.bishop,
church,
minist
and preach
, as well as a large number of churchmen such as
Richard Bancroft and John Whitgift (see Figure 3). Topic 8 includes the words
publish
, poem
, and play,
along with poets like Robert
Herrick, John Donne, and John Dryden.
We then analyzed the frequency of our estimated relationships between people who do and do not share topics. For all three topic models, estimated relationships between people who shared a topic are more frequent than between-topic estimated relationships; the specific results for the ten-topic model can be seen in Figure 4. This coincides with the expectation for homophily (also known as assortativity) and a qualitative, semantics-based reading of the same data: book authors are more likely to be linked to other notable authors as opposed to notable military personnel. While running a topic model with different parameters (i.e. number of topics) changes the specific results, within-topic relationships remain more frequent than between-topic relationships. We therefore concluded that the Poisson Graphical Lasso produces results compatible with other, semantically-sensitive methods.
A table of relationship confidence estimates in the ten-topic modelOur second validation step was to compare our results with alternative methods of constructing a confidence matrix. Using Spearman correlations, which measure how well the ordering of two ranked lists align, we evaluated how each method performed against expert-generated ranked relationship lists. We had earlier considered using three possible methods for inferring a correlation matrix from the document-count matrix: 1) ranking by simple correlation (high positive correlations are higher ranked relationships); 2) running the Poisson Graphical Lasso and ranking edges by the value of ϱ in which the edge was added to the model (edges added with more penalization are higher ranked relationships); and 3) running the Poisson Graphical Lasso and ranking edges by the value of the regression coefficient (higher positive coefficients are higher ranked relationships). According to statistical theory, both versions of the Poisson Graphical Lasso should perform as well as, if not better than, simple correlation because of their ability to screen off friend-of-a-friend connections, as described in section three above. We hoped to find this reflected in our Spearman correlations, in order to conclude that our approach produces results compatible with statistical theory.
We chose to test this on James Harrington and John Milton by taking the top
thirty relationships according to each of these three methods and combining them
to create a master list of thirty and eighty relationships, respectively.
Faculty and PhD students with backgrounds in the early modern period were given
the combined names in random order and asked, first, to rank the relationships
according to a question we used to approximate relationship importance,
specifically how unhappy would experts be if this relationship were not
included among the main actor’s top relationships?
and, second, to mark
the relationships as true/false. Despite only being an approximation to
relationship importance, the ranking list still proved far more difficult for
the humanists to generate than the true/false list.
We wanted to choose the statistical method that created lists most closely
correlated to the humanists’ list, as measured by Spearman correlation. The
Spearman correlations of each method were extremely similar in the humanists’
ranked lists and – combined with humanists’ concerns over producing the list in
the first place – led us to abandon the effort to optimize our algorithm for the
order of ranks. Instead, we attempted to determine which method obtained more
correct relationships – that is, relationships humanists marked as true – in the
top k estimated connections. For analysis of James
Harrington’s thirty connections, all three methods performed similarly; for John
Milton’s eighty connections, using simply the correlation coefficient led to
worse estimates earlier on, confirming that the Poisson Graphical Lasso can more
accurately reproduce sections of the network than correlation alone.
Lastly, for our third validation step, we wanted to evaluate the accuracy of our
final inferred network, in comparison to the relational knowledge conveyed by a
humanist reading of the ODNB. We therefore chose twelve people from the network
and calculated the precision and recall rates for their relationships. The
twelve people were not a random sample. Rather, they were chosen to represent a
variety of conditions within our dataset, including gender, number of estimated
relationships, deduplicated names, and appearance within individual vs. shared
ODNB biographies. Some of these conditions are relatively rare within the
dataset on the whole. For each person, we checked their inferred edges from
40-100% confidence – qualitatively tagged as our possible
to
certain
confidence interval – against a list of associations manually
compiled from the ODNB documents by reading through each person’s biographical
entry and other entries in which their name appears.
Together, these twelve people had twenty-eight relationships in our likely-to-certain (60-100) confidence interval, of which three were incorrect, leading to an 89.29% precision rate (see Figure 5). Expanding our confidence interval to also include possible relationships (40-100) – in other words, sacrificing precision to increase recall – gave us one hundred and seven relationships of which twenty-seven were incorrect, leading to a still-respectable 74.77% precision rate. The majority of these false positives were caused by specific conditions within our data: group biographies, duplicate names, and an abnormally high percentage of co-mentions within related biographies. Removing the four people who satisfied these specific conditions from our sample left us with fifty relationships and a 86.00% precision rate in our 40-100 confidence interval, which suggests that many of the errors in our dataset are associated with people who fulfill these conditions, which impaired our algorithm’s ability to correctly capture their relationships via co-mentions. Because our validation sample had taken care to include some of our most problematic case-types, even though instances of some of those case-types are relatively few, we deemed these measures of precision adequate as a starting point for further curation of the network via crowd-sourcing on our website at www.sixdegreesoffrancisbacon.com.
Precision and Recall for a Subset of the Inferred NetworkCalculating a global recall – the fraction of desired results obtained from the ODNB as a whole – on our dataset would have required us to identify connections across the entire biographical corpus of the ODNB, a prohibitively labor-intensive process when done manually. We therefore calculated two partial measures of recall instead. The first measure is article-level recall – that is, a measure of the ability of our network to capture the same relationships as a human reading a specific biographical article. By this measure, our recall numbers were low, with our 40-100 confidence interval including only 25.74% of the relationships mentioned in the article. Low article recall can be attributed, at least in part, to two factors: first, the decision to impose a five-mentions threshold during the NER stage, which excludes infrequently mentioned names about which the ODNB provides insufficient network data, and, second, the way some names are mentioned in the ODNB, which prevented them from being picked up by NER.
Next, we calculated the measure we call SDFB
recall – that is, the ability
of our computer algorithms to infer relationships for the subset of people
mentioned in a specific biographical article who were also included in our
overall network. This adjustment – excluding people who did not pass the
five-mentions threshold or were not captured by NER – leads to a significantly
higher recall numbers, at 51.49%, again for the 40-100 confidence interval.
Further expansion of the confidence interval to 10-100 increases the SDFB recall
rate to 67.33%, showing that within the subset of names captured by NER and
included in our node dataset, high recall rates can be achieved at the lowest
confidence intervals. Though higher recall rates would of course be desirable in
theory, we deemed it preferable to have a relatively accurate but sparse network
rather than a full but error-ridden network, and further increases in recall
would require corresponding trade-offs in precision.
Though the map of the early modern social network created by our inference
procedures is far from perfect, it provides a sizeable base of persons and
relationships that can be gradually corrected and expanded to encompass the
interests of a wide range of humanist scholars. This network can also be
examined, validated, refined, and expanded by scholars, students, and other
end-users through a dynamic wiki front-end with sophisticated network
visualization tools. We consider such an approach complementary to several
successful approaches that focus on smaller subsets of society
The questions humanists care most about often turn on documentary evidence of connections, and immersion in an archive or a published collection of letters yields qualitative knowledge of unparalleled depth and richness. Yet the humanities would need to see massive investments in historical analysis, palaeography, languages, and other humanistic research skills in order to investigate anything close to the number of relationships inferred using our model. Since little in the current funding climate suggests that such investments are immediately forthcoming, the promises of historical network analysis would remain unrealized in the absence of a different approach. Hence our probabilistic network inferences, which create a workable infrastructure for subsequent investigation. Instead of starting the process of mapping the network from scratch, we remediate existing scholarship for further addition, expansion, development, and correction.
The time, moreover, appears to be right. With open access research gaining
momentum, and more and more texts entering the public domain, probabilistic
text-mining approaches afford wider lenses and present new opportunities
At the same time, partnerships between traditional small-scale projects and larger-scale projects like
Our approach isn’t just a new method. It yields substantive insights as well.
Applying quantitative network measures like network degree has allowed us to
identify interesting figures, such as those who have relatively high degrees but
who don’t have ODNB entries of their own. An analysis of high-degree nodes
without ODNB entries shows an intriguingly high representation of schoolmasters
and publishers. Individuals like Thomas Smelt, an ardent royalist who taught at
the Northallerton Free School in Yorkshire, and Edward Sylvester, who ran a
grammar school in Oxford, were not deemed significant enough to warrant full
biographical entries, but they are nevertheless key nodes connecting those who
were
It is also possible from this work to understand more about non-British people who figure prominently the life of the nation. Scholars can learn much about international dimensions by attending to the frequency of non-native names appearing frequently in the ODNB. Our five-mention threshold also helps us see gender differences in a revealing light. The cultural practice of changing one’s surname at marriage means that women face particular obstacles meeting our artificially-imposed five-mention threshold. In several cases, men appear in the dataset simply because they are mentioned in association with important women – wives, sisters, or mothers who for various reasons may not themselves appear in the dataset. The woman referred to in the ODNB as
Ultimately, our work with the ODNB has shown that processing an entire corpus of documents and running a statistical procedure is computationally feasible with the resources generally available to university scholars. We have also shown that it is possible to implement a statistical approach that infers a validated social network. While not all highest-confidence edges are among the strongest identified by experts – and some expert-identified relationships are not near the top edges found – there is enough overall validation on many classes of relationships to suggest our method is viable for reconstructing historical social networks, within a reasonable margin of error, from large textual corpora.
This process admittedly has several shortcomings, especially from the
perspective of humanists for whom margin of error
is a less than
reassuring phrase. Absent further research, there is no surefire way to
determine whether a given confidence estimate accurately reflects the current
state of scholarship (as represented by the ODNB) or is instead an artifact of
the bespoke model we developed. Nor are relationships in the resulting dataset
typed
– friends and enemies remain functionally identical in our
results, though the difference of course matters decisively in real life. Proof
or other evidence about a given relationship will initially appear elusive: the
process yields few clues about where to start researching a relationship –
though our crowd-sourcing website does at least provide users links to ODNB and
JSTOR articles that mention both people in a relationship. And humanists must be
involved at every stage for validation, interpretation, de-duping, and
disambiguation. However, the end result of this process is of demonstrable use
to experts in early modern Britain and it is likely extensible to other large
corpora.
We are the first to acknowledge that our network inference procedure comes freighted with assumptions and technical limitations that may pose obstacles to its transferability to other social networks generated from other data sources. Inferring a network from biographical texts requires assuming that the co-occurrence of names in a document is a reasonable predictor of a relationship between the named persons. Although we believe this is a reasonable and productive assumption for ODNB texts, it is not an equally reasonable assumption for all data sources. Network inference will be only as good as the NER on which it depends. Differences in NER availability and accuracy for different languages (Stanford, for example, has separate modules for Spanish, German, and Chinese), as well as differences in naming conventions across cultures, time periods, discourses, and biographical data may decrease its effectiveness, though NER can be tuned for different datasets. Because the ODNB entries have been carefully edited and checked, they are relatively error free, but projects that aim to mine biographical reference works that exist only in uncorrected Optical Character Recognition documents will begin with a significant level of textual error. Those who seek to employ our procedures on other biographical data sources should perform checks to ensure that it is inferring edges between nodes at level of accuracy that they deem acceptable.
While our interest has been in reconstructing the social network of a specific time and place – sixteenth- and seventeenth-century Britain – there are few barriers to re-deploying our method in other historical or contemporary societies. We used short biographical entries, but we could with minor changes have used contemporary book prefaces, modern scholarly articles, blogs, or other kinds of texts. All that is needed is machine-readable text in which the co-occurrence of names is a reasonable indicator of connections between persons. Future work on our specific project may thus involve expanding the collection of documents used in our network. Target documents currently include the publishing data in the English Short Title Catalog and the prefatory material in Early English Books Online. We would also aim to incorporate datasets whose strengths would mitigate the data’s current weaknesses, such as collections of letters written by women or urban apprenticeship rolls.
We have also begun to expand our network through the data provided by individual scholars via our website interface at www.sixdegreesoffrancisbacon.com. To encourage mass integration of other datasets, we have incorporated features into our website to allow the tagging of nodes and the visualization of sub-networks by those tags. However, we also have a particular interest in scholars adding citations to confirm our statistically predicted relationships, as well enriching those relationships by providing information about their type and timespan. Our ultimate goal is to create a versatile and extensible network that people interested in all aspects of early modern Britain – including the scholarship on early modern Britain – can use for their research, as well as to pioneer a general technique of creating social networks from texts that other scholars can apply to other periods and societies.
Our statistical approach follows the model of G.I. Allen and Z. Liu p nodes and E is the set of pairwise edges. We
relate the graph to a random vector Y=(Y1, ... ,
Yp) by requiring that for each non-edge
(j,k)∉E, the variables Yj and Yk are conditionally independent given all the
remaining variables Y∖{j,k}, where ∖{j,k} denotes
the complement V∖{j,k}. Commonly, Y=(Y1, ... ,
Yp) is assumed to follow a multivariate normal
distribution Np(µ,∑), in which case pairwise
conditional independence holds if and only if ∑jk-1=0 -1.
If we have n independent and identically distributed
observations of Y, we can employ penalized likelihood methods, where we
place a one-norm penalty on elements of the concentration matrix. This
penalized likelihood can be maximized efficiently for large p using a graphical lasso j on the remaining variables Y∖j=Y∖{j}. We estimate σjk-1 = 0 if the estimated regression coefficients
of variable j on k or k on j are estimated to be 0
For count data like ours the normality assumption may be inappropriate and a
modification of the above methods was developed by Allen and Liu for Poisson
graphical models, in which the relationships between nodes are defined by a
conditional Poisson distribution
The Poisson Markov random field implied by this relationship is not amenable
to inferring network structures, as it requires θjk ≤ 0 for all pairs {j,k}
and combine the implied local relationships into a network structure.
We can then view θij as a measure of relationship
strength between i and j. In
Allen and Liu, the model is fit using the Poisson Graphical Lasso – a
penalized regression method similar to the graphical lasso j’s counts
on the rest. That is, for each node we solve the following:
Here ϱ is a matrix of penalty parameters and ⋆ denotes component-wise
multiplication. An edge is determined to exist between nodes j and k if θ̂ jk > 0 and/or
The motivating data for Allen and Liu are the RNA-sequencing measurements
from p genes in n experiments;
their goal is to determine which genes are connected
to each other in
a metabolic process p names in n
biographies; our goal is to determine which historical figures had
"connections" to each other in a variety of social contexts. Two modeling
considerations unique to this type of data and practical objective, which
lead us to slight modifications in method, are the variance of document
lengths and the irrelevance of negative edge estimates.
Documents in the ODNB vary greatly in length. People tend to have longer
biographies when biographers know more about them or have deemed them
historically significant. Allen and Liu note that it is important to
normalize the data to be approximately independent and identically
distributed Poisson random variables, since their model is sensitive to
deviations from this assumption
As a preliminary test of this method, we calculate the Spearman correlation between lists of relationships provided by humanities scholars and
For our test set, simple correlation fails first, while those for our model – with and without sectioning – remain similar. Sectioning fails to improve correlation on some historical actors, but it leads to slight improvements in correlation for others.
Furthermore, when fitting the model, a large fraction of θ̂ jk values are negative. When this
coefficient is negative, it does not make sense to estimate a resulting
edge, since negative coefficients imply a negative relationship between the
counts of name
We want to be able to attach a confidence estimate to all edges (which can
be used to rank connections), instead of just obtaining a yes or no estimate
for each potential edge. Let the matrix C represent a symmetric confidence
matrix (where each entry Cjk = Ckj = confidence attached to edge existing between
person j and k). An informal
confidence estimate can be obtained by refitting the model many times on
random subsets of the data and computing the fraction of models in which a
specific edge is found in the model.
The method of estimating the final edge confidences is as follows:
Note that θ̂ (t)jk is the estimate for the coefficient on
the t
There are a number of methods described in the literature for selecting the
tuning parameter ϱ. When the goal is prediction of the response variable,
cross-validation is a natural choice. When the goal is network inference –
specifically, we want to know whether each edge is in
or out
–
stability selection can be used instead, as is done in Allen and Liu
Our confidence estimates for a specific value of ϱ correspond to a single
point on the stability paths mentioned in Meinshausen and Bühlmann stable variables
– or in this case, edges –
does not matter significantly. In our experiments with values of ϱ ranging
from 0.001 (many edges) to 100 (no edges), we also find the confidence
estimates tend to not vary too much for different reasonable values of ϱ,
where reasonable is defined as a low enough penalty such that variables are
actually added, but high enough penalty so that the algorithm converges
rapidly.
Different people sometimes have the same names, and disambiguating them is difficult. When name duplication only happens rarely, it may be feasible to disambiguate manually. However, there are no less than twelve John Smith’s and ten Archibald Campbell’s in our node set; overall nearly a thousand names refer to multiple people. Furthermore, many of the people with these names overlap in lifespans, including a large number of parents who gave their own names to their children.
To process these duplicate names, we use a twofold method. First, we employ chronological filters on all our potential relationship edges. Two people cannot have a relationship if their lifespans did not overlap. We do, however, allow a one-year margin of error so that posthumous children may still have edges to their biological fathers. For people with unknown birth and death dates, we allow a twenty-year span before and after their known period of activity. For people for whom only a birth or a death date is known, we allow for up to a 110-year lifespan, erring on the side of inclusivity rather than exclusivity.
In the cases where there is chronological overlap in the lifespans of people with duplicate names, we fall back on probabilities. If the name was generated by NER, we evenly split the mentions among each of the people with that name – that is, we assign them each an equal probability. However, if our duplicates all have biographical entries, we assign each person a probability based on the length of their biography. This serves as an approximation of the relative frequency we expect each person to appear in the overall ODNB, which we use to weight the mentions accordingly.
For example, Francis Walsingham, the principal secretary, has a biography
that is 30 times the length of Francis Walsingham, the Jesuit. Therefore we
argue a mention of Francis Walsingham in some other ODNB biography is 30
times more likely to refer to the former rather than the latter. To follow
this logic through, we would assign weights of 97% to the principal
secretary and 3% to the Jesuit. Yet we don’t want to obscure the
lesser-known Jesuit so thoroughly. Therefore, we cap the percentages at a
max/min of 75% and 25% so that someone with an extremely long biography
cannot dominate the probabilities completely. Thus in the period of overlap
between their two lifespans, 75% of the instances of Francis
Walsingham
are attached to the principal secretary and 25% are
attached to the Jesuit. In practice, this does yield lower confidence
estimates and more false positives for split-mention
nodes’
relationships, but we consider this an acceptable as a starting point for
further, manual curation.
Prospectively, after enough humanists contribute their expert knowledge to
the network via our crowd-sourcing website, it will be possible to use their
contributions to refine our inference model by making local changes to the
penalty parameter. We could do this by allowing the penalty matrix, ϱ, to
vary for different relationships. If our experts confirm a relationship
between actors j and k, we set
ϱjk=ϱkj=0, which
usually ensures that θ̂ jk,
Further information on how we generated our network can be found, along with
our R code, at: https://github.com/sdfb/sdfb_network