An introduction to latent semantic analysis

Latent Semantic Analysis (LSA) is a theory and method for extracting and representing the contextual-usage meaning of words by statistical computations applied to a large corpus of text (Landauer and Dumais, 1997). The underlying idea is that the aggregate of all the word contexts in which a given word does and does not appear provides a set of mutual constraints that largely determines the similarity of meaning of words and sets of words to each other. The adequacy of LSA’s reflection of human knowledge has been established in a variety of ways. For example, its scores overlap those of humans on standard vocabulary and subject matter tests; it mimics human word sorting and category judgments; it simulates word–word and passage–word lexical priming data; and, as reported in 3 following articles in this issue, it accurately estimates passage coherence, learnability of passages by individual students, and the quality and quantity of knowledge contained in an essay. Introduction to Latent Semantic Analysis 3 An Introduction to Latent Semantic Analysis Research reported in the three articles that follow—Foltz, Kintsch & Landauer (1998/this issue), Rehder, et al. (1998/this issue), and Wolfe, et al. (1998/this issue)—exploits a new theory of knowledge induction and representation (Landauer and Dumais, 1996, 1997) that provides a method for determining the similarity of meaning of words and passages by analysis of large text corpora. After processing a large sample of machine-readable language, Latent Semantic Analysis (LSA) represents the words used in it, and any set of these words—such as a sentence, paragraph, or essay—either taken from the original corpus or new, as points in a very high (e.g. 50-1,500) dimensional “semantic space”. LSA is closely related to neural net models, but is based on singular value decomposition, a mathematical matrix decomposition technique closely akin to factor analysis that is applicable to text corpora approaching the volume of relevant language experienced by people. Word and passage meaning representations derived by LSA have been found capable of simulating a variety of human cognitive phenomena, ranging from developmental acquisition of recognition vocabulary to word-categorization, sentence-word semantic priming, discourse comprehension, and judgments of essay quality. Several of these simulation results will be summarized briefly below, and additional applications will be reported in detail in following articles by Peter Foltz, Walter Kintsch, Thomas Landauer, and their colleagues. We will explain here what LSA is and describe what it does. LSA can be construed in two ways: (1) simply as a practical expedient for obtaining approximate estimates of the contextual usage substitutability of words in larger text segments, and of the kinds of—as yet incompletely specified— meaning similarities among Introduction to Latent Semantic Analysis 4 words and text segments that such relations may reflect, or (2) as a model of the computational processes and representations underlying substantial portions of the acquisition and utilization of knowledge. We next sketch both views. As a practical method for the characterization of word meaning, we know that LSA produces measures of word-word, word-passage and passage-passage relations that are well correlated with several human cognitive phenomena involving association or semantic similarity. Empirical evidence of this will be reviewed shortly. The correlations demonstrate close resemblance between what LSA extracts and the way peoples’ representations of meaning reflect what they have read and heard, as well as the way human representation of meaning is reflected in the word choice of writers. As one practical consequence of this correspondence, LSA allows us to closely approximate human judgments of meaning similarity between words and to objectively predict the consequences of overall word-based similarity between passages, estimates of which often figure prominently in research on discourse processing. It is important to note from the start that the similarity estimates derived by LSA are not simple contiguity frequencies, co-occurrence counts, or correlations in usage, but depend on a powerful mathematical analysis that is capable of correctly inferring much deeper relations (thus the phrase “Latent Semantic”), and as a consequence are often much better predictors of human meaning-based judgments and performance than are the surface level contingencies that have long been rejected (or, as Burgess and Lund, 1996 and this volume, show, unfairly maligned) by linguists as the basis of language phenomena. LSA, as currently practiced, induces its representations of the meaning of words and passages from analysis of text alone. None of its knowledge comes directly from perceptual information about the physical world, from instinct, or from experiential intercourse with bodily functions, feelings and intentions. Thus its representation of reality is bound to be somewhat sterile and bloodless. However, it does take in descriptions and verbal outcomes of all these juicy processes, and so far as writers have put such things into Introduction to Latent Semantic Analysis 5 words, or that their words have reflected such matters unintentionally, LSA has at least potential access to knowledge about them. The representations of passages that LSA forms can be interpreted as abstractions of “episodes”, sometimes of episodes of purely verbal content such as philosophical arguments, and sometimes episodes from real or imagined life coded into verbal descriptions. Its representation of words, in turn, is intertwined with and mutually interdependent with its knowledge of episodes. Thus while LSA’s potential knowledge is surely imperfect, we believe it can offer a close enough approximation to people’s knowledge to underwrite theories and tests of theories of cognition. (One might consider LSA's maximal knowledge of the world to be analogous to a well-read nun’s knowledge of sex, a level of knowledge often deemed a sufficient basis for advising the young.) However, LSA as currently practiced has some additional limitations. It makes no use of word order, thus of syntactic relations or logic, or of morphology. Remarkably, it manages to extract correct reflections of passage and word meanings quite well without these aids, but it must still be suspected of resulting incompleteness or likely error on some occasions. LSA differs from some statistical approaches discussed in other articles in this issue and elsewhere in two significant respects. First, the input data "associations" from which LSA induces representations are between unitary expressions of meaning—words and complete meaningful utterances in which they occur—rather than between successive words. That is, LSA uses as its initial data not just the summed contiguous pairwise (or tuple-wise) co-occurrences of words but the detailed patterns of occurrences of very many words over very large numbers of local meaning-bearing contexts, such as sentences or paragraphs, treated as unitary wholes. Thus it skips over how the order of words produces the meaning of a sentence to capture only how differences in word choice and differences in passage meanings are related. Introduction to Latent Semantic Analysis 6 Another way to think of this is that LSA represents the meaning of a word as a kind of average of the meaning of all the passages in which it appears, and the meaning of a passage as a kind of average of the meaning of all the words it contains. LSA's ability to simultaneously—conjointly—derive representations of these two interrelated kinds of meaning depends on an aspect of its mathematical machinery that is its second important property. LSA assumes that the choice of dimensionality in which all of the local wordcontext relations are simultaneously represented can be of great importance, and that reducing the dimensionality (the number parameters by which a word or passage is described) of the observed data from the number of initial contexts to a much smaller—but still large—number will often produce much better approximations to human cognitive relations. It is this dimensionality reduction step, the combining of surface information into a deeper abstraction, that captures the mutual implications of words and passages. Thus, an important component of applying the technique is finding the optimal dimensionality for the final representation. A possible interpretation of this step, in terms more familiar to researchers in psycholinguistics, is that the resulting dimensions of description are analogous to the semantic features often postulated as the basis of word meaning, although establishing concrete relations to mentalisticly interpretable features poses daunting technical and conceptual problems and has not yet been much attempted. Finally, LSA, unlike many other methods, employs a preprocessing step in which the overall distribution of a word over its usage contexts, independent of its correlations with other words, is first taken into account; pragmatically, this step improves LSA’s results considerably. However, as mentioned previously, there is another, quite different way to think about LSA. Landauer and Dumais (1997) have proposed that LSA constitutes a fundamental computational theory of the acquisition and representation of knowledge. They maintain that its underlying mechanism can account for a long-standing and important mystery, the inductive property of learning by which people acquire much more knowledge Introduction to Latent Semantic Analysis 7 than appears to be available in experience, the infamous problem of the "insufficiency of evidence" or "poverty of the stimulus." The LSA mechanism that solves the problem consists simply of accommodating a very large number of local co-occurrence relations (between the right kinds of observational units) simultaneously in a space of the right