Introduction

I have been recently reviewing and rewriting a paper for publication that I first wrote back in 2011. The paper (Wallis forthcoming) concerns the problem of how we test whether repeated runs of the same experiment obtain essentially the same results, i.e. results are not significantly different from each other.

These meta-tests can be used to test an experiment for replication: if you repeat an experiment and obtain significantly different results on the first repetition, then, with a 1% error level, you can say there is a 99% chance that the experiment is not replicable.

These tests have other applications. You might be wishing to compare your results with those of others in the literature, compare results with different operationalisation (definitions of variables), or just compare results obtained with different data – such as comparing a grammatical distribution observed in speech with that found within writing.

The design of tests for this purpose is addressed within the t-testing ANOVA community, where tests are applied to continuously-valued variables. The solution concerns a particular version of an ANOVA, called “the test for interaction in a factorial analysis of variance” (Sheskin 1997: 489).

However, anyone using data expressed as discrete alternatives (A, B, C etc) has a problem: the classical literature does not explain what you should do.

Gradient and point tests

gradient-point-test

Figure 1: Point tests (A) and gradient tests (B), from Wallis (forthcoming).

The rewrite of the paper caused me to distinguish between two types of tests: ‘point tests’, which I describe below, and ‘gradient tests’.

These tests can be used to compare results drawn from 2 × 2 or r × c χ² tests for homogeneity (also known as tests for independence). This is the most common type of contingency test, which can be computed using Fisher’s exact method or as a Newcombe-Wilson difference interval.

  • A gradient test (B) evaluates whether the gradient or difference between point 1 and point 2 differs between runs of an experiment, dp₁ – p₂. This concerns whether claims about the rate of change, or size of effect, observed are replicable. Gradient tests can be extended, with increasing degrees of freedom, into tests comparing patterns of effect.
  • A point test (A) simply asks whether data at either point, evaluated separately, differs between experimental runs. This concerns whether single observations, such as p₁, are replicable. Point tests can be extended into ‘multi-point’ tests, which we discuss below.

Point tests only apply to homogeneity data. If you wish to compare outcomes from goodness of fit tests, you need a version of the gradient test, to compare differences from an expected Pdp₁ – P. Since different data sets may have different expected P, a distinct ‘point test for goodness of fit’ would be meaningless.

The earlier version of the paper, which has been published on this blog since its launch 2012, focused on gradient tests. The possibility of carrying out a point test was mentioned in passing. In this blog post I want to focus on point tests.

The obvious problem with gradient tests is that two experimental runs might obtain the same gradient but in fact be very different in start and end points. Consider the following graph.

equal-gradient

Figure 2: Why we need two different types of test: (almost) equal gradients but unequal points.

Point tests

The data in Figure 1 is calculated from two 2 × 2 tables drawn from a paper by Aarts, Close and Wallis (2013).

Note: To obtain Figure 2, I simply replaced one frequency in the first table: 46 with 100. The data is also found on the 2×2 homogeneity tab in this Excel spreadsheet, which contains a wide range of separability tests.

To make our exposition clearer, Table 1 uses the same format as in the Excel spreadsheet (with the dependent variable distributed vertically) rather than the format in the paper.

spoken LLC
(1960s)
ICE-GB
(1990s)
Total
shall 124 46 170
will 501 544 1,045
Total 625 590 1,215
written LOB
(1960s)
FLOB
(1990s)
Total
shall 355 200 555
will 2,798 2,723 5,521
Total 3,153 2,923 6,076

Frequency data for the choice modal shall out of the choice shall vs. will, various sources, from Aarts et al. (2013).

Aarts et al. carried out 2 × 2 homogeneity tests for the two tables separately. These test whether modal shall declines as a proportion of the modal shall/will alternation between the two time points. In other words, we compare LLC with ICE-GB data, and LOB with FLOB data.

To carry out a point test we simply rotate the test 90 degrees, e.g. to compare data at the 1960s point we compare LLC with LOB.

As I have explained elsewhere (Wallis 2013), there are a number of different methods for carrying out this comparison.

These include:

  1. The z test for two independent proportions (Sheskin 1997: 226).
  2. The Newcombe-Wilson interval test (Newcombe 1998).
  3. The 2 × 2 χ² test for homogeneity (independence).

These are all standard tests and each is discussed in papers and elsewhere on this blog.

The advantage of the third approach is that it is extensible to c-way multinomial observations by using a 2 × c χ² test.

The multi-point test

The tests listed above can be used to compare the 1960s and 1990s intervals in Figure 1 separately.

However, in many cases it would be helpful to have a method that evaluated both pairs of observations in a single test. This can be generalised to a series of r observations. To do this, in (Wallis forthcoming) I propose what I call a multi-point test.

We generalise the χ² formula by summing over i = 1..r:

  • χd² = ∑χ²(i)

where χ²(i) represents the χ² score for homogeneity for each set of data at position i in the distribution.

This test has r × df(i) degrees of freedom, where df(i) is the degrees of freedom for each χ² point test. So, in the worked example we have seen, the summed test has two degrees of freedom:

spoken LLC
(1960s)
ICE-GB
(1990s)
Total
shall 124 46 170
will 501 544 1,045
Total 625 590 1,215
written LOB
(1960s)
FLOB
(1990s)
Total
shall 355 200 555
will 2,798 2,723 5,521
Total 3,153 2,923 6,076
χ² 34.6906 0.6865 35.3772

Applying the generalised point test calculation to the table above. χ² = 35.38 is significant with 2 degrees of freedom and α = 0.05.

Since the computation sums independently-calculated χ² scores, each score may be individually considered for significant difference (with df(i) degrees of freedom). Hence we can see above the large score for the 1960s data (individually significant) and the small score for 1990s (individually non-significant).

Note: Whereas χ² is generally associative (non-directional), the summed equation (χd²) is not. Nor is this computation the same as a 3 dimensional test (t × r × c). Variables are treated differently.

  • The multi-point test factors out variation between tests over the independent variable (in this instance: time). This means that if there is a lot more data in one table at a particular time period, this fact does not skew the results.
  • On the other hand, it does not factor out variation over the dependent variable – after all, this is precisely what we wish to examine!

Naturally, like the point test, this test may be generalised to multinomial observations.

A Newcombe-Wilson multi-point test

An alternative multi-point test for binomial (two-way) variables employs a sum of χ² values abstracted from Newcombe-Wilson tests.

  1. Carry out Newcombe-Wilson tests for each point test i at a given error level α, obtaining Di, Wi⁻ and Wi⁺.
  2. Identify the inner interval width Wi for each test:
    • if D< 0, Wi = Wi⁻; WiWi⁺ otherwise.
  3. Use the difference Di and inner interval Wi to compute χ² scores:
    • χ²(i) = (Di . zα/2 / Wi)².

It is then possible to sum χ²(i) as before.

Using the data in the worked example we obtain:

1960s: Di = 0.0858, Wi⁻ = -0.0347 and Wi⁺ = 0.0316 (significant).
1990s: Di = 0.0095, Wi⁻ = -0.0194 and Wi⁺ = 0.0159 (ns).

Since Di is positive in both cases, we use the upper interval width each time. This gives us χ² scores of 28.4076 and 1.3769 respectively, which obtains a sum of 29.78. Compared to the first method above, this approach tends to downplay extreme differences.

In conclusion

The point test and the additive generalisation of this test into a ‘multi-point test’ represent a method of contrasting multiple runs of the same experiment, comparing observed changes in different subcorpora or genres, or examine the empirical effect of changing definitions of variables.

These tests consider the null hypothesis that individual observations are not different; or, in the multi-point case, that in general the observations are not different.

  • They do not evaluate the gradient between points or the size of effect. If we wish to compare sizes of effect we would need to use one of the methods for this purpose described in (Wallis forthcoming).
  • The method only applies to comparing tests for homogeneity (independence). To compare goodness of fit data, a different approach is required (also described in Wallis forthcoming).

Nonetheless, these tests are useful meta-tests that build on classical Pearson χ² tests, and they are useful tools in our analytical armoury.

See also

References

Sheskin, D.J. 1997. Handbook of Parametric and Nonparametric Statistical Procedures. Boca Raton, Fl: CRC Press.

Newcombe, R.G. 1998. Interval estimation for the difference between independent proportions: comparison of eleven methods. Statistics in Medicine 17: 873-890.

Wallis, S.A. 2013. z-squared: the origin and application of χ². Journal of Quantitative Linguistics 20:4, 350-378. » Post

Wallis, S.A. forthcoming (first published 2011). Comparing χ² tables for separability of distribution and effect. London: Survey of English Usage. » Post

Detecting direction in interaction evidence

IntroductionPaper (PDF)

I have previously argued (Wallis 2014) that interaction evidence is the most fruitful type of corpus linguistics evidence for grammatical research (and doubtless for many other areas of linguistics).

Frequency evidence, which we can write as p(x), the probability of x occurring, concerns itself simply with the overall distribution of a linguistic phenomenon x – such as whether informal written English has a higher proportion of interrogative clauses than formal written English. In order to calculate frequency evidence we must define x, i.e. decide how to identify interrogative clauses. We must also pick an appropriate baseline n for this evaluation, i.e. we need to decide whether to use words, clauses, or any other structure to identify locations where an interrogative clause may occur.

Interaction evidence is different. It is a statistical correlation between a decision that a writer or speaker makes at one part of a text, which we will label point A, and a decision at another part, point B. The idea is shown schematically in Figure 1. A and B are separate ‘decision points’ in a given relationship (e.g. lexical adjacency), which can be also considered as ‘variables’.

Figure 1: Associative inference from lexico-grammatical choice variable A to variable B (sketch).

Figure 1: Associative inference from lexico-grammatical choice variable A to variable B (sketch).

This class of evidence is used in a wide range of computational algorithms. These include collocation methods, part-of-speech taggers, and probabilistic parsers. Despite the promise of interaction evidence, the majority of corpus studies tend to consist of discussions of frequency differences and distributions.

In this paper I want to look at applications of interaction evidence which are made more-or-less at the same time by the same speaker/writer. In such circumstances we cannot be sure that just because B follows A in the text, the decision relating to B was made after the decision at A. Continue reading

Adapting variance for random-text sampling

Introduction Paper (PDF)

Conventional stochastic methods based on the Binomial distribution rely on a standard model of random sampling whereby freely-varying instances of a phenomenon under study can be said to be drawn randomly and independently from an infinite population of instances.

These methods include confidence intervals and contingency tests (including multinomial tests), whether computed by Fisher’s exact method or variants of log-likelihood, χ², or the Wilson score interval (Wallis 2013). These methods are also at the core of others. The Normal approximation to the Binomial allows us to compute a notion of the variance of the distribution, and is to be found in line fitting and other generalisations.

In many empirical disciplines, samples are rarely drawn “randomly” from the population in a literal sense. Medical research tends to sample available volunteers rather than names compulsorily called up from electoral or medical records. However, provided that researchers are aware that their random sample is limited by the sampling method, and draw conclusions accordingly, such limitations are generally considered acceptable. Obtaining consent is occasionally a problematic experimental bias; actually recruiting relevant individuals is a more common problem.

However, in a number of disciplines, including corpus linguistics, samples are not drawn randomly from a population of independent instances, but instead consist of randomly-obtained contiguous subsamples. In corpus linguistics, these subsamples are drawn from coherent passages or transcribed recordings, generically termed ‘texts’. In this sampling regime, whereas any pair of instances in independent subsamples satisfy the independent-sampling requirement, pairs of instances in the same subsample are likely to be co-dependent to some degree.

To take a corpus linguistics example, a pair of grammatical clauses in the same text passage are more likely to share characteristics than a pair of clauses in two entirely independent passages. Similarly, epidemiological research often involves “cluster-based sampling”, whereby each subsample cluster is drawn from a particular location, family nexus, etc. Again, it is more likely that neighbours or family members share a characteristic under study than random individuals.

If the random-sampling assumption is undermined, a number of questions arise.

  • Are statistical methods employing this random-sample assumption simply invalid on data of this type, or do they gracefully degrade?
  • Do we have to employ very different tests, as some researchers have suggested, or can existing tests be modified in some way?
  • Can we measure the degree to which instances drawn from the same subsample are interdependent? This would help us determine both the scale of the problem and arrive at a potential solution to take this interdependence into account.
  • Would revised methods only affect the degree of certainty of an observed score (variance, confidence intervals, etc.), or might they also affect the best estimate of the observation itself (proportions or probability scores)?

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.

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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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Choice vs. use

Introduction

Many linguistic researchers are interested in semasiological variation, that is, how the meaning of words and expressions may be observed to vary over time or space. One word might have one dominant meaning or use at one point in time, and other meanings may supplant them. This is of obvious interest to etymology. How do new meanings come about? Why do others decline? Do old meanings die away or retain a specialist use?

Most of the research we have discussed on this blog is, by contrast, concerned with onomasiological variation, or variation in the choice of words or expressions to express the same meaning. In a linguistic choice experiment, the field of meaning is held to be constant, or approximately so, and we are concerned primarily with language production:

  • Given that a speaker (or writer, but we take speech as primary) wishes to express some thought, T, what is the probability that they will use expression E₁ out of the alternate forms {E₁, E₂,…} to express it?

This probability is meaningful in the language production process: it measures the actual use out of the options available to the speaker, at the point of utterance.

Conversely, semasiological researchers are concerned with a different type of probability:

  • Given that a speaker used an expression E, what is the probability that their meaning was T₁ out of the set of {T₁, T₂,…}?

For the hearer, this measure can also be thought of as the exposure rate: what proportion of times should a hearer (reader) interpret E as expressing T₁? This probability is meaningful to a language receiver, but it is not a meaningful statistic at the point of language production.

From the speaker’s point of view we can think of onomasiological variation as variation in choice, and semasiological variation as variation in relative proportion of use.

Continue reading

Reciprocating the Wilson interval

Introduction

How can we calculate confidence intervals on a property like sentence length (as measured by the number of words per sentence)?

You might want to do this to find out whether or not, say, spoken utterances consist of shorter or longer sentences than those found in writing.

The problem is that the average number of words per sentence is not a probability. If you think about it, this ratio will (obviously) equal or exceed 1. So methods for calculating intervals on probabilities won’t work without recalibration.

Aside: You are most likely to hit this type of problem if you want to plot a graph of some non-probabilistic property, or you wish to cite a property with an upper and lower bound for some reason. Sometimes expressing something as a probability does not seem natural. However, it is a good discipline to think in terms of probabilities, and to convert your hypotheses into hypotheses about probabilities as far as possible. As we shall see, this is exactly what you have to do to apply the Wilson score interval.

Note also that just because you want to calculate confidence intervals on a property, you also have to consider whether the property is freely varying when expressed as a probability.

The Wilson score interval (w⁻, w⁺), is a robust method for computing confidence intervals about probabilistic observations p.

Elsewhere we saw that the Wilson score interval obtained an accurate approximation to the ‘exact’ Binomial interval based on an observed probability p, obtained by search. It is also well-constrained, so that neither upper nor lower bound can exceed the probabilistic range [0, 1].

But the Wilson interval is based on a probability. In this post we discuss how this method can be used for other quantities.

Continue reading