I’m a relative newcomer to digital humanities; I’ve been doing this for about a year now. The content of the field has been interesting, but in some ways even more interesting is the way it has transformed my perception of the academy as a social structure. There are clearly going to be debates over the next few years between more and less digitized humanists, and debate is probably a good thing for everyone. But the debate can be much more illuminating if we acknowledge up front that it’s also a tension between two different forms of social organization.
Here’s what happens when that dimension of the issue goes unacknowledged: a tenured or tenure-track faculty member will give a talk or write a blog post about the digital humanities, saying essentially “you’ve got some great tools there, but before they can really matter, their social implications need to be theorized more self-consciously.” Said professor is then surprised when the librarians, or academic professionals, or grad students, who have in many cases designed and built those tools reply with a wry look.
I hasten to add that I’ve got nothing against theories. I wouldn’t mind constructing a few myself. Literary theory, social theory, statistical theory — they’re all fun. But when the word “Theory” is used without adjective or explication, it does in my view deserve a wry look. When you take away all the adjectives, what’s left is essentially a status marker.
So let’s not play that game. Nothing “needs to be theorized” in a vague transitive way; academics who use phrases like that need to realize what they’re saying. DH is an intensely interdisciplinary field that already juggles several different kinds of theory, and actively reflects on the social significance of its endeavors (e.g. in transforming scholarly communication). It is also, among other things, an insurgent challenge to academic hierarchy, organized and led by people who often hold staff positions — which means that the nature of the boundary between practice and theory is precisely one of the questions it seeks to contest.
But as long as everyone understands that “theory” is not a determinate object belonging to a particular team, then I say, the more critique, debate, and intellectual exchange the better. For instance, I quite enjoyed Natalia Cecire’s recent blog post on ways DH could frame its engagement with literary theory more ambitiously. I don’t know whether it’s a good idea to have a “theory THATcamp”; I haven’t been to THATcamp, and don’t know whether its strengths (which seem to lie in collaboration) are compatible with that much yacking. But I think Cecire is absolutely right to insist that DH can and should change the way the humanities are practiced. Because digital approaches make it possible to ask and answer different kinds of questions, there’s going to be a reciprocal interaction between humanistic goals and digital methods, not, as Cecire puts it, a “merely paratactic, additive concatenation.” We’re going to need to theorize about methods and goals at the same time. Together. Intransitively.
[Sun, Oct 23, 2011 — This post is slightly revised from the original version, mostly for clarity.]
Right now Latent Semantic Analysis is the analytical tool I’m finding most useful. By measuring the strength of association between words or groups of words, LSA allows a literary historian to map themes, discourses, and varieties of diction in a given period.
This approach, more than any other I’ve tried, turns up leads that are useful for me as a literary scholar. But when I talk to other people in digital humanities, I rarely hear enthusiasm for it. Why doesn’t LSA get more love? I see three reasons:
But for a literary historian, the value of this technique does not depend on its claim to identify synonyms and antonyms. We may actually be more interested in contingent associations (e.g., “sensibility” — “rousseau” in the list on the left) than we are in the core “meaning” of a word.
I’ll return in a moment to this point. It has important implications, because it means that we want LSA to do something slightly different than linguists and information scientists have designed it to do. The “flaws” they have tried to iron out of the technique may not always be flaws for our purposes.
2. People who do topic-modeling may feel that they should use more-recently-developed Bayesian methods, which are supposed to be superior on theoretical grounds. I’m acknowledging this point just to set it aside; I’ve mused out loud about it once already, and I don’t want to do more musing until I have rigorously compared the two methods. I will say that from the perspective of someone just getting started, LSA is easier to implement than Bayesian topic modeling: it runs faster and scales up more easily.
3. The LSA algorithm provided by an off-the-shelf package is not necessarily the best algorithm for a literary historian. At bottom, that’s why I’m writing this post: humanists who want to use LSA are going to need guidance from people in their own discipline. Computer scientists do acknowledge that LSA requires “tuning, which is viewed as a kind of art.”  But they also offer advice about “best practices,” and some of those best practices are defined by disciplinary goals that humanists don’t share.
For instance, the power of LSA is often said to come from “reducing the dimensionality of the matrix.” The matrix in question is a term-document matrix — documents are listed along one side of the matrix, and terms along the other, and each cell of the matrix (tfi,j) records the number of times term i appears in document j, modified by a weighting algorithm described at the end of this post.
That term-document matrix in and of itself can tell you a lot about the associations between words; all you have to do is measure the similarity between the vectors (columns of numbers) associated with each term. But associations of this kind won’t always reveal synonyms. For instance, “gas” and “petrol” might seem unrelated, because they substitute for each other in different sociolects and are rarely found together. To address that problem, you can condense the matrix by factorizing it with a technique called singular value decomposition (SVD). I’m not going to get into the math here, but the key is that condensing the matrix partially fuses related rows and columns — and as a result, the compressed matrix is able to measure transitive kinds of association. The words “gas” and “petrol” may rarely appear together. But they both appear with the same kinds of other words. So when dimensionality reduction “merges” the rows representing similar documents, “gas” and “petrol” will end up being strongly represented in the same merged rows. A compressed matrix is better at identifying synonyms, and for that reason at information retrieval. So there is a lot of consensus among linguists and information scientists that reducing the number of dimensions in the matrix is a good idea.
But literary historians approach this technique with a different set of goals. We care a lot about differences of sociolect and register, and may even be more interested in those sources of “noise” than we are in purely semantic relations. “Towering,” for instance, is semantically related to “high.” But I could look that up in a dictionary; I don’t need a computer program to tell me that! I might be more interested to discover that “towering” belongs to a particular subset of poetic diction in the eighteenth century. And that is precisely the kind of accident of distribution that dimensionality-reduction is designed to filter out. For that reason, I don’t think literary applications of LSA are always going to profit from the dimensionality-reduction step that other disciplines recommend.
For about eight months now, I’ve been using a version of LSA without dimensionality reduction. It mines associations simply by comparing the cosine-similarity of term vectors in a term-document matrix (weighted in a special way to address differences of document size). But I wanted to get a bit more clarity about the stakes of that choice, so recently I’ve been comparing it to a version of LSA that does use SVD to compress the matrix.
Here’s a quick look at the results. (I’m using 2,193 18c volumes, mostly produced by TCP-ECCO; volumes that run longer than 100,000 words get broken into chunks that can range from 50k-100k words.) In many cases, the differences between LSA with and without compression are not very great. In the case of “delicacy,” for instance, both algorithms indicate that “delicate” has the strongest association. “Politeness” and “tenderness” are also very high on both lists. But compare the second row. The algorithm with compression produces “sensibility” — a close synonym. On the left-hand side, we have “woman.” This is not a synonym for “delicacy,” and if a linguist or computer scientist were evaluating these algorithms, it would probably be rejected as a mistake. But from a literary-historical point of view, it’s no mistake: the association between “delicacy” and femininity is possibly the most interesting fact about the word.
In short, compressing the matrix with SVD highlights semantic relationships at the cost of slightly blurring other kinds of association. In the case of “delicacy,” the effect is fairly subtle, but in other cases the difference between the two approaches is substantial. For instance, if you measure the similarity of term vectors in a matrix without compression, “high” and “towering” look entirely different. The main thing you discover about “high” is that it’s used for physical descriptions of landscape (“lies,” “hills”), and the main thing you discover about “towering” is that it’s used in poetic contexts (“flowery,” “glittering”).
In a matrix that has undergone dimensionality reduction with SVD, associations have a much more semantic character, although they are still colored by other dimensions of context. Which of these two algorithms is more useful for humanistic purposes? I think the answer is going to depend on the goals being pursued in a given research project — if you’re interested in “topics” that are strictly semantic, you might want to use an algorithm that reduces dimensionality with SVD. If you’re interested in discourses, sociolects, genres, or types of diction, you might use LSA without dimensionality reduction.
My purpose here isn’t to choose between those approaches; it’s just to remind humanists that the algorithms we borrow from other disciplines are often going to need to be customized for our own disciplinary purposes. Information scientists have designed topic-modeling algorithms that produce semantically unified topics, because semantic categorization is important for them. But in literary history, we also care about other dimensions of language, and we don’t have to judge topic-modeling algorithms by strictly semantic criteria. How should we judge them? It will probably take decades for us to answer that question fully, but the short answer is just — by how well, in practice, they help us locate critically and historically interesting patterns.
A couple of technical notes: A fine point of LSA that can matter a great deal is how you weight the individual cells in the term-document matrix. For the normal LSA algorithm that uses dimensionality reduction, the consensus is that “log-entropy weighting” works well. You take the log of each frequency, and multiply the whole term vector by the entropy of the vector. I have found that this also works well for humanistic purposes.
For LSA without dimensionality reduction, I would recommend weighting cells by subtracting the expected frequency from the observed frequency. This formula “evens the playing field” between common and uncommon words — and it does so, vitally, in a way that gives a word’s absence from a long document more weight than its absence from a short one. (Much of LSA’s power actually comes from learning where a given word tends not to appear. ) I have tried various ways of applying log-entropy weighting without compressing the matrix, and I do not recommend it. Those two techniques belong together.
For reasons that remain somewhat mysterious (although the phenomenon itself is widely discussed), dimensionality reduction seems to work best when the number of dimensions retained is in the range of 250-350. Intuitively, it would seem possible to strike a sort of compromise between LSA methods that do and don’t compress the matrix by reducing dimensionality less drastically (perhaps only, say, cutting it by half). But in practice I find that doesn’t work very well; I suspect compression has to reach a certain threshold before the noise inherent in the process starts to cancel itself out and give way to a new sort of order.
 Thomas K. Landauer, Peter W. Foltz, and Darrell Latham, An Introduction to Latent Semantic Analysis, Discourse Processes 25 (1998): 259-84. Web reprint, p. 22.
 Preslav Nakov, Elena Valchanova, and Galia Angelova, “Towards Deeper Understanding of Latent Sematic Analysis Performance,” Recent Advances in Natural Language Processing, ed. Nicolas Nicolov (Samokov, Bulgaria: John Benjamins, 2004), 299.
 Landauer, Foltz, and Latham, p. 24.
I’ve been posting mostly about collections built by other people (TCP-ECCO and Google). But I’m also in the process of building a small (thousand-title) 19c collection myself, in collaboration with E. Jordan Sellers. Jordan is selecting titles for the collection; I’m writing the Python scripts that process the texts. This is a modest project intended to support research for a few years, not a model for long-term curatorial practice. But we’ve encountered a few problems specific to the early 19c, and I thought I might share some of our experience and tools in case they’re useful for other early-19c scholars.
I originally wanted to create a larger collection, containing twenty or thirty thousand volumes, on the model of Ben Schmidt’s impressive work with nineteenth-century volumes vacuumed up from the Open Library. But because I needed a collection that bridged the eighteenth and nineteenth centuries, I found I had to proceed more slowly. The eighteenth century itself wasn’t the problem. Before 1800, archaic typography makes most optical character recognition unreliable — but for that very reason, TCP-ECCO has been producing clean, manually-keyed versions of 18c texts, enough at least for a small collection. The later 19c also isn’t a problem, because after 1830 or so, OCR quality is mostly adequate.
But between 1800 and (say) 1830, you fall between two stools. It’s technically the nineteenth century, so people assume that OCR ought to work. But in practice, volumes from this period still have a lot of eighteenth-century typographical quirks, including loopy ligatures, the notorious “long s,” and worn or broken type. So the OCR is often pretty vile. I’m willing to put up with background noise if it’s evenly distributed. But these errors are distributed unevenly across the lexicon and across time, so they could actually distort conclusions if left unaddressed.
I decided to build a Python script to do post-processing correction of OCR. There are a lot of ways to do this; my approach was modeled on a paper written by Thomas A. Lasko and Susan E. Hauser for the National Library of Medicine. Briefly, what they show is that OCR correction becomes much more reliable when the program is given statistical information about the language, and errors, to be expected in a given domain. They’re working with contemporary text, but the principle holds even more strongly when you’re working in a different historical period. A generic spellchecker won’t perform well with texts that contain period spellings (“despatch,” “o’erflow’d”), systematic f/s substitution, and a much higher proportion of Latin and French than we’re used to. If your system corrects every occurrence of “même” to “mime,” you’re going to end up with a surprising number of mimes; if you accept “foul” at face value as a correctly-spelled word, you’re going to have very little “soul” in your collection.
Briefly, I customized my spellchecker for the early 19c in three ways:
• The underlying dictionary included period spellings as well as common French and Latin terms, and recorded the frequency of each term in the 18/19c domain. I used frequencies (lightly) to guide fuzzy matching.
• To calculate “edit distance,” I used a weighted matrix that recorded the probability of specific character substitutions in early-19c OCR, learning as it went along.
• To resolve pairs like “foul/soul” and “flip/slip/ship,” where common OCR errors produce a token that could also be a real word, I extracted 2gram frequencies from the Google ngram database so that the program could judge which word made more sense in context. I.e., in the case of “the flip sailed,” the program can infer that a word before “sailed” is pretty likely to be “ship.”
A few other tricks are needed to optimize speed, and to make sure the script doesn’t over-correct proper nouns; anyone who’s interested in doing this should drop me a line for a fuller description and a copy of the code.
The results aren’t perfect, but they’re good enough to be usable (I am also recording the number of corrections and uncorrectable tokens so that I can assess margins of error later on).
I haven’t packaged this code yet for off-the-shelf use; it’s still got a few trailing wires. But if you want to cannibalize/adapt it, I’d be happy to give you a copy. Perhaps more importantly, I’d like to share a couple of sets of rules that might be helpful for anyone who’s attempting to normalize an 18/19c collection. Both of these rulesets are tab-delimited utf-8 .txt files. First, my list of 4600 rules for correcting 18/19c spellings, including syncopated past-tense forms like “bury’d” and “drop’d.” (Note that syncope cannot always be fixed simply by adding back an “e.” Rules for normalizing poetic syncope — “flow’ry,” “ta’en” — are clustered at the end of the file, so you can delete them if desired.) This ruleset has been transformed by a long series of joins and filtering operations, and edited manually, but I should acknowledge that part of the original list was borrowed from the source files that accompany WordHoard, developed at Northwestern University. I should also warn potential users that these rules are designed to normalize spelling to modern British practice.
The other thing it might be useful to share is a list of 2grams extracted from the Google English corpus, that I use for contextual spellchecking. This includes only 2grams where one of the two elements is a token like “fix” or “flip” that could be read either as a valid word or as an OCR error caused by the long s. Since the long s is also a problem in the Google dataset itself up to 1820, this list was based on frequencies from 1825-50. That’s not perfect for correcting texts in the 1800-1820 period, but I find that in practice it’s adequate. There are two columns here: the 2gram itself, and the frequency.