The other end of the telescope

Introduction

The standard approach to teaching (and thus thinking about) statistics is based on projecting distributions of ranges of expected values. The distribution of an expected value is a set of probabilities that predict what the value will be, according to a mathematical model of what you predict should happen.

For the experimentalist, this distribution is the imaginary distribution of very many repetitions of the same experiment that you may have just undertaken. It is the output of a mathematical model.

  • Note that this idea of a projected distribution is not the same as the term ‘expected distribution’. An expected distribution is a series of values you predict your data should match.
  • Thus in what follows we simply compare a single expected value P with an observed value p. This can be thought of as comparing the expected distribution E = {P, 1 – P} with the observed distribution O = {p, 1 – p}.

Thinking about this projected distribution represents a colossal feat of imagination: it is a projection of what you think would happen if only you had world enough and time to repeat your experiment, again and again. But often you can’t get more data. Perhaps the effort to collect your data was huge, or the data is from a finite set of available data (historical documents, patients with a rare condition, etc.). Actual replication may be impossible for material reasons.

In general, distributions of this kind are extremely hard to imagine, because they are not part of our directly-observed experience. See Why is statistics difficult? for more on this. So we already have an uphill task in getting to grips with this kind of reasoning.

Significant difference (often shortened to ‘significance’) refers to the difference between your observations (the ‘observed distribution’) and what you expect to see (the expected distribution). But to evaluate whether a numerical difference is significant, we have to take into account both the shape and spread of this projected distribution of expected values.

When you select a statistical test you do two things:

  • you choose a mathematical model which projects a distribution of possible values, and
  • you choose a way of calculating significant difference.

The problem is that in many cases it is very difficult to imagine this projected distribution, or — which amounts to the same thing — the implications of the statistical model.

When tests are selected, the main criterion you have to consider concerns the type of data being analysed (an ‘ordinal scale’, a ‘categorical scale’, a ‘ratio scale’, and so on). But the scale of measurement is only one of several parameters that allows us to predict how random selection might affect the resampling of data.

A mathematical model contains what are usually called assumptions, although it might be more accurate to call them ‘preconditions’ or parameters. If these assumptions about your data are incorrect, the test is likely to give an inaccurate result. This principle is not either/or, but can be thought of as a scale of ‘degradation’. The less the data conforms to these assumptions, the more likely your test is to give the wrong answer.

This is particularly problematic in some computational applications. The programmer could not imagine the projected distribution, so they tweaked various parameters until the program ‘worked’. In a ‘black-box’ algorithm this might not matter. If it appears to work, who cares if the algorithm is not very principled? Performance might be less than optimal, but it may still produce valuable and interesting results.

But in science there really should be no such excuse.

The question I have been asking myself for the last ten years or so is simply can we do better? Is there a better way to teach (and think about) statistics than from the perspective of distributions projected by counter-intuitive mathematical models (taken on trust) and significant tests? Continue reading

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, unreliably annotated. This issue frequently arises in corpora to which an algorithm has been applied, but where the results have not been checked by linguists, a situation which is unavoidable with mega-corpora. However, this is a general problem. 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, to ensure 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 corpora such as ICE-GB and DCPSE, do not include modal semantics in their annotation scheme. In such cases the issue 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 (see Aarts et al. 2013), but that choice is conditioned by the semantic content of the utterance.

Continue reading

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.
Continue reading

EDS Resources

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

Session 15: Introduction to statistics

Sessions 18 and 19: Statistics Workshops

Suggested further reading

An unnatural probability?

Not everything that looks like a probability is.

Just because a variable or function ranges from 0 to 1, it does not mean that it behaves like a unitary probability over that range.

Natural probabilities

What we might term a natural probability is a proper fraction of two frequencies, which we might write as p = f/n.

  • Provided that f can be any value from 0 to n, p can range from 0 to 1.
  • In this formula, f and n must also be natural frequencies, that is, n stands for the size of the set of all cases, and f the size of a true subset of these cases.

This natural probability is expected to be a Binomial variable, and the formulae for z tests, χ² tests, Wilson intervals, etc., as well as logistic regression and similar methods, may be legitimately applied to such variables. The Binomial distribution is the expected distribution of such a variable if each observation is drawn independently at random from the population (an assumption that is not strictly true with corpus data).

Another way of putting this is that a Binomial variable expresses the number of individual events of Type A in a situation where an outcome of either A and B are possible. If we observe, say 8 out of 10 cases are of Type A, then we can say we have an observed probability of A being chosen, p(A | {A, B}), of 0.8. In this case, f is the frequency of A (8), and n the frequency of both A and B (10). See Wallis (2013a). Continue reading

Comparing frequencies within a discrete distribution

Note:
This page explains how to compare observed frequencies f₁ and f₂ from the same distributionF = {f₁, f₂,…}. To compare observed frequencies f₁ and f₂ from different distributions, i.e. where F₁ = {f₁,…} and F₂ = {f₂,…}, you need to use a chi-square or Newcombe-Wilson test.

Introduction

In a recent study, my colleague Jill Bowie obtained a discrete frequency distribution by manually classifying cases in a small sample drawn from a large corpus.

Jill converted this distribution into a row of probabilities and calculated Wilson score intervals on each observation, to express the uncertainty associated with a small sample. She had one question, however:

How do we know whether the proportion of one quantity is significantly greater than another?

We might use a Newcombe-Wilson test (see Wallis 2013a), but this test assumes that we want to compare samples from independent sources. Jill’s data are drawn from the same sample, and all probabilities must sum to 1. Instead, the optimum test is a dependent-sample test.

Example

A discrete distribution looks something like this: F = {108, 65, 6, 2}. This is the frequency data for the middle column (circled) in the following chart.

This may be converted into a probability distribution P, representing the proportion of examples in each category, by simply dividing by the total: P = {0.60, 0.36, 0.03, 0.01}, which sums to 1.

We can plot these probabilities, with Wilson score intervals, as shown below.

tag1cmp

An example graph plot showing the changing proportions of meanings of the verb think over time in the US TIME Magazine Corpus, with Wilson score intervals, after Levin (2013). In this post we discuss the 1960s data (circled). The sum of each column probability is 1. Many thanks to Magnus for the data!

So how do we know if one proportion is significantly greater than another?

  • When comparing values diachronically (horizontally), data is drawn from independent samples. We may use the Newcombe-Wilson test, and employ the handy visual rule that if intervals do not overlap they must be significantly different.
  • However, probabilities drawn from the same sample (vertically) sum to 1 — which is not the case for independent samples! There are k−1 degrees of freedom, where k is the number of classes. It turns out that the relevant significance test we need to use is an extremely basic test, but it is rarely discussed in the literature.

Continue reading

Capturing patterns of linguistic interaction

Abstract Full Paper (PDF)

Numerous competing grammatical frameworks exist on paper, as algorithms and embodied in parsed corpora. However, not only is there little agreement about grammars among linguists, but there is no agreed methodology for demonstrating the benefits of one grammar over another. Consequently the status of parsed corpora or ‘treebanks’ is suspect.

The most common approach to empirically comparing frameworks is based on the reliable retrieval of individual linguistic events from an annotated corpus. However this method risks circularity, permits redundant terms to be added as a ‘solution’ and fails to reflect the broader structural decisions embodied in the grammar. In this paper we introduce a new methodology based on the ability of a grammar to reliably capture patterns of linguistic interaction along grammatical axes. Retrieving such patterns of interaction does not rely on atomic retrieval alone, does not risk redundancy and is no more circular than a conventional scientific reliance on auxiliary assumptions. It is also a valid experimental perspective in its own right.

We demonstrate our approach with a series of natural experiments. We find an interaction captured by a phrase structure analysis between attributive adjective phrases under a noun phrase with a noun head, such that the probability of adding successive adjective phrases falls. We note that a similar interaction (between adjectives preceding a noun) can also be found with a simple part-of-speech analysis alone. On the other hand, preverbal adverb phrases do not exhibit this interaction, a result anticipated in the literature, confirming our method.

Turning to cases of embedded postmodifying clauses, we find a similar fall in the additive probability of both successive clauses modifying the same NP and embedding clauses where the NP head is the most recent one. Sequential postmodification of the same head reveals a fall and then a rise in this additive probability. Reviewing cases, we argue that this result can only be explained as a natural phenomenon acting on language production which is expressed by the distribution of cases on an embedding axis, and that this is in fact empirical evidence for a grammatical structure embodying a series of speaker choices.

We conclude with a discussion of the implications of this methodology for a series of applications, including optimising and evaluating grammars, modelling case interaction, contrasting the grammar of multiple languages and language periods, and investigating the impact of psycholinguistic constraints on language production.

Continue reading