Coping with imperfect data

Introduction

One of the challenges for corpus linguists is that many of the distinctions that we wish to make are either not annotated in a corpus at all or, if they are represented in the annotation, the quality of the annotation may be imperfect.

This frequently arises in corpora to which an algorithm has been applied, but the results have not been checked by linguists. However, we would always recommend that cases be reviewed for accuracy of annotation.

A version of this issue also arises when checking for the possibility of alternation, that is that items of Type A can be replaced by Type B items, and vice-versa. An example might be epistemic modal shall vs. will. Most corpora, including richly-annotated copora such as ICE-GB and DCPSE, do not include modal semantics in their annotation scheme. In such cases the key point is not that the annotation is “imperfect”, rather that our experiment relies on a presumption that the speaker has the choice of either type at any observed point, and that we may wish to disentangle effects of variation in the use of epistemic and root meanings (Aarts et al. 2013).

Suppose you have preliminary results, obtained with a rough-and-ready search, and these obtain a significant result. However you suspect that there are some errors in the annotation, or that some cases that have been included in your search (or excluded from your search) should not be counted. What do you do?

  1. Is it worth manually checking every example, of which there may be hundreds, if not thousands?
  2. Do you do nothing, i.e. rely on the results you have so far?
  3. Or is there a third option?

One of the advantages of obtaining a mathematical understanding of significance tests and confidence intervals, which this blog attempts to convey, is that it offers us a mathematical answer to this problem. Continue reading

Is language really “a set of alternations?”

The perspective that the study of linguistic data should be driven by studies of individual speaker choices has been the subject of attack from a number of linguists.

The first set of objections have come from researchers who have traditionally focused on linguistic variation expressed in terms of rates per word, or per million words.

No such thing as free variation?

As Smith and Leech (2013) put it: “it is commonplace in linguistics that there is no such thing as free variation” and that indeed multiple differing constraints apply to each term. On the basis of this observation they propose an ‘ecological’ approach, although in their paper this approach is not clearly defined.

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Summer School in English Corpus Linguistics 2014

I am pleased to announce that we will be running our second Summer School in English Corpus Linguistics at University College London from Monday 7 to Wednesday 9 July 2014.

Last year’s course was highly successful, and we had extremely positive feedback from those who attended.

Places are limited, and will be allocated on a first-come, first-served basis.

Summer School class

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Binomial → Normal → Wilson

Introduction

One of the questions that keeps coming up with students is the following.

What does the Wilson score interval represent, and why is it the right way to calculate a confidence interval based around an observation? 

In this blog post I will attempt to explain, in a series of hopefully simple steps, how we get from the Binomial distribution to the Wilson score interval. I have written about this in a more ‘academic’ style elsewhere, but I haven’t spelled it out in a blog post.
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EDS Resources

This post contains the resources for students taking the UCL English Linguistics MA, all in one place.

Session 11

Session 12

Suggested further reading

Genre differences and experimental observations

Spoken categories, modal verbs and change over time

In a recently-published paper, Bowie, Wallis and Aarts (2013) demonstrate that observations regarding changes in the frequency of modal verbs over time are highly sensitive to differences in genre (‘register’ or ‘text category’). Our paper, although based on spoken British English, may shed some light on a recent dispute between Leech (2011) and Millar (2009) regarding how linguists should interpret corpus observations regarding changes in the modal verb system in written US English.

The following table summarises statistically significant percentage decreases and increases of individual modal verbs as a proportion of the number of tensed verb phrases (VPs that could conceivably take a modal verb), within different spoken genre subcategories of the Diachronic Corpus of Present-day Spoken English (DCPSE). The statistical test used examines differences in observed probabilities between samples, i.e. a Newcombe-Wilson test.

For our purposes the cited percentages do not matter, but the direction of travel (indicated by coloured cells) does.

can may could might shall will should would must All
formal f2f ns ns ns ns ns ns -60% ns -75%
informal f2f 27% -42% ns 47% -32% ns ns ns -53% ns
telephone -37% ns -44% ns -56% -30% ns -44% ns -35%
b. discussions -41% -59% ns ns -83% ns ns ns -54% -20%
b. interviews ns -61% ns -59% ns -41% -55% -32% -57% -35%
commentary ns ns ns ns -93% 58% ns ns -64% ns
parliament ns ns ns ns ns -39% ns -30% ns -20%
legal x-exam 304% ns ns ns ns ns 1,265% 254% ns 157%
spontaneous ns ns ns ns ns ns ns ns ns ns
prepared sp. ns -63% ns ns ns 327% ns -32% -48% ns
All genres ns -40% -11% ns -48% 13% -14% -7% -54% -6%

Significant changes (α<0.05) in the proportion of individual core modals out of tensed verb phrases from the 1960s (LLC) to 1990s (ICE-GB) components in DCPSE, adapted from Bowie et al. 2013.

This study concerns modal verbs within text categories. Against a general baseline (words, verb phrases or tensed verb phrases), the total number of modals decrease in use over the course of the period covered by the data (at least, noting the caveat, for spoken English data sampled comparably). Above, we employ tensed verb phrases as the most meaningful baseline out of the three. See That vexed problem of choice.

  • Note that if we take all genres together (bottom row in the table), except for will, every significant change is a decline in use, but in the (large) category of informal face-to-face conversation (second row from top), can and might are both significantly increasing.
  • Legal cross-examination is a predictable outlier, but broadcast interviews and discussions appear to generate very different results. Continue reading