ma in english linguistics experimental design and statistics sean wallis survey of english usage...

MA in English LinguisticsMA in English LinguisticsExperimental design and statisticsExperimental design and statistics

Sean WallisSurvey of English Usage

University College London

[email protected]

OutlineOutline

• What is a research question?

• Choice and baselines

• Making sense of probability

• Observing change in a corpus

• Drawing inferences to larger populations

• Estimating error in observations

• Testing results for significance

What is a research question?What is a research question?

• You may have heard this phrase last term

• What do you think we mean by a “research question”?

• Can you think of any examples?

ExamplesExamples

• Some example research questions

ExamplesExamples


– smoking is good for you

ExamplesExamples



– dropped objects accelerate toward the ground at 9.8 metres per second squared

ExamplesExamples




– ’s is a clitic rather than a word

ExamplesExamples





– the word shall is used less often in recent

years

ExamplesExamples





– the word shall is used less often in recent years

– the degree of preference for shall rather than will has declined in British English over the period 1960s-1990s

Testable hypothesesTestable hypotheses

• An hypothesis = a testable research question

• Compare– the word shall is used less in recent years

to– the degree of preference for shall rather

than will has declined in British English over the period 1960s-1990s

• How could you test these hypotheses?

Questions of choiceQuestions of choice

• Suppose we wanted to test the following hypothesis using DCPSE

– the word shall is used less in recent years

• When we say the word shall is used less...

– ...less compared to what?• traditionally corpus linguists have “normalised” data as

a proportion of words (so we might say shall is used less frequently per million words)

• But what might this mean?


• From the speaker’s perspective:– The probability of a speaker using a word like shall

depends on whether they had the opportunity to say it in the first place

– They were about to say will, but said shall instead





– Per million words might still be relevant from the hearer’s perspective






• If we can identify all points where the choice arose, we have an ideal baseline for studying linguistic choices made by speakers/writers.


• From the speaker’s perspective:– The probability of a speaker using a word like shall depends

on whether they had the opportunity to say it in the first place



• If we can identify all points where the choice arose, we have an ideal baseline for studying linguistic choices made by speakers/writers.

– Can all cases of will be replaced by shall ?– What about second or third person shall ?

BaselinesBaselines

• The baseline is a central element of the hypothesis– Changes are always relative to something– You can get different results with different baselines– Different baselines imply different conclusions

• We have seen two different kinds of baselines– A word baseline

• shall per million words– A choice baseline (an “alternation experiment”)

• shall as a proportion of the choice shall vs. will (including’ll ), when the choice arises

BaselinesBaselines

• In many cases it is very difficult to identify all cases where “the choice” arises– e.g. studying modal verbs

BaselinesBaselines


• You may need to pick a different baseline– Be as specific as you can

• words VPs tensed VPs alternating modals

BaselinesBaselines




alternation = “different

words, same meaning”

BaselinesBaselines




• Other hypotheses imply different baselines:– Different meanings of the same word:

• e.g. uses of very, as a proportion of all cases of veryvery +N - the very personvery +ADJ - the very tall personvery +ADV - very slightly moving

alternation = “different

words, same meaning”

semasiological

variation}

ProbabilityProbability

• We are used to concepts like these being expressed as numbers:– length (distance, height)– area– volume– temperature – wealth (income, assets)


• We are used to concepts like these being expressed as numbers:– length (distance, height)– area– volume– temperature – wealth (income, assets)

• We are going to discuss another concept:– probability (proportion, percentage)


• Based on another, even simpler, idea:– probability p = x / n


• Based on another, even simpler, idea:– probability p = x / n – e.g. the probability that the

speaker says will instead of shall



• where– frequency x (often, f )

• the number of times something actually happens• the number of hits in a search

– e.g. the probability that the speaker says will instead of shall





– cases of will






– baseline n is• the number of times something could happen• the number of hits

– in a more general search – in several alternative patterns (‘alternate forms’)

– cases of will








– cases of will

– total: will + shall








• Probability can range from 0 to 1

– e.g. the probability that the speaker says will instead of shall– cases of will

– total: will + shall

A simple research questionA simple research question

• What happens to modal shall vs. will over time in British English?– Does shall increase or decrease?

• What do you think?

• How might we find out?

Lets get some dataLets get some data

• Open DCPSE with ICECUP– FTF query for first person declarative shall:

• repeat for will

Lets get some dataLets get some data

• Open DCPSE with ICECUP– FTF query for first person declarative shall:

• repeat for will– Corpus Map:

• DATE Do the first set of queries and then drop into Corpus

Map}

Modal Modal shallshall vs. vs. willwill over time over time

• Plotting probability of speaker selecting modal shall out of shall/will over time (DCPSE)

0.0

0.2

0.4

0.6

0.8

1.0

1955 1960 1965 1970 1975 1980 1985 1990 1995

p(shall | {shall, will})

(Aarts et al., 2013)

shallshall = 100% = 100%



• Plotting probability of speaker selecting modal shall out of shall/will over time (DCPSE)

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0.6

0.8

1.0

1955 1960 1965 1970 1975 1980 1985 1990 1995


Is shall going up or down?




Is Is shall shall going up or down? going up or down?

• Whenever we look at change, we must ask ourselves two things:

Is Is shall shall going up or down? going up or down? • Whenever we look at change, we must ask ourselves two things:

What is the change relative to?– What is our baseline for comparison? – In this case we ask

• Does shall decrease relative to shall +will ?

Is Is shall shall going up or down? going up or down? • Whenever we look at change, we must ask ourselves two things:

What is the change relative to?– What is our baseline for comparison? – In this case we ask

• Does shall decrease relative to shall +will ?

How confident are we in our results?– Is the change big enough to be reproducible?

The ‘sample’ and the The ‘sample’ and the ‘population’‘population’• The corpus is a sample


• If we ask questions about the proportions of certain words in the corpus– We ask questions about the sample– Answers are statements of fact



• Now we are asking about “British English”

?



• Now we are asking about “British English”– We want to draw an inference

• from the sample (in this case, DCPSE)• to the population (similarly-sampled BrE utterances)

– This inference is a best guess– This process is called inferential statistics

Basic inferential Basic inferential statisticsstatistics

• Suppose we carry out an experiment– We toss a coin 10 times and get 5 heads– How confident are we in the results?

• Suppose we repeat the experiment• Will we get the same result again?

Basic inferential Basic inferential statisticsstatistics

• Suppose we carry out an experiment– We toss a coin 10 times and get 5 heads– How confident are we in the results?

• Suppose we repeat the experiment• Will we get the same result again?

• Let’s try…– You should have one coin– Toss it 10 times– Write down how many heads you get– Do you all get the same results?

The Binomial distributionThe Binomial distribution

• Repeated sampling tends to form a Binomial distribution around the expected mean X

F

N = 1

x

531 7 9

• We toss a coin 10 times, and get 5 heads

X



F

N = 4

x

531 7 9

• Due to chance, some samples will have a higher or lower score

X



F

N = 8

x

531 7 9


X



F

N = 12

x

531 7 9


X



F

N = 16

x

531 7 9


X



F

N = 20

x

531 7 9


X



F

N = 26

x

531 7 9


X

The Binomial distributionThe Binomial distribution• It is helpful to express x as the probability of choosing a head, p, with expected mean P

• p = x / n– n = max. number of

possible heads (10)

• Probabilities are inthe range 0 to 1=percentages

(0 to 100%)

F

p

0.50.30.1 0.7 0.9

P


• Take-home point:– A single observation, say x hits (or p as a

proportion of n possible hits) in the corpus, is not guaranteed to be correct ‘in the world’!

• Estimating the confidence you have in your results is essential

F

p

P

0.50.30.1 0.7 0.9

p


• Take-home point:– A single observation, say x hits (or p as a

proportion of n possible hits) in the corpus, is not guaranteed to be correct ‘in the world’!

• Estimating the confidence you have in your results is essential

– We want to makepredictions about future runs of the same experiment

F

p

P

p

0.50.30.1 0.7 0.9

Binomial Binomial Normal Normal

• The Binomial (discrete) distribution is close to the Normal (continuous) distribution

x

F

0.50.30.1 0.7 0.9


• Any Normal distribution can be defined by only two variables and the Normal function z

z . S z . S

F

– With more data in the experiment, S will be smaller

p0.50.30.1 0.7

population

mean P

standard deviationS = P(1 – P) / n


• Any Normal distribution can be defined by only two variables and the Normal function z

z . S z . S

F

2.5% 2.5%

population

mean P

– 95% of the curve is within ~2 standard deviations of the expected mean

standard deviationS = P(1 – P) / n

p0.50.30.1 0.7

95%

– the correct figure is 1.95996!

= the critical value of z for an error level of 0.05.

The single-sample The single-sample zz test...test...

• Is an observation p > z standard deviations from the expected (population) mean P?

z . S z . S

F

P0.25% 0.25%

p0.50.30.1 0.7

observation p• If yes, p is

significantly different from P

...gives us a “confidence ...gives us a “confidence interval”interval”• P ± z . S is the confidence interval for P

– We want to plot the interval about p

z . S z . S

F

P0.25% 0.25%

p0.50.30.1 0.7

...gives us a “confidence ...gives us a “confidence interval”interval”• P ± z . S is the confidence interval for P

– We want to plot the interval about p

w+

F

P0.25% 0.25%

p0.50.30.1 0.7

observation p

w–

...gives us a “confidence ...gives us a “confidence interval”interval”• The interval about p is called the

Wilson score interval

• This interval reflects the Normal interval about P:

• If P is at the upper limit of p,p is at the lower limit of P

(Wallis, 2013)

F

P0.25% 0.25%

p

w+

observation p

w–

0.50.30.1 0.7


• Simple test: – Compare p for

• all LLC texts in DCPSE (1956-77) with• all ICE-GB texts (early 1990s)

– We get the following data

– We may plot the probabilityof shall being selected,with Wilson intervals

LLC ICE-GB totalshall 110 40 150will 78 58 136total 188 98 286

0.0

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0.8

1.0

LLC ICE-GB



• Simple test: – Compare p for

• all LLC texts in DCPSE (1956-77) with• all ICE-GB texts (early 1990s)

– We get the following data

– We may plot the probabilityof shall being selected,with Wilson intervals

0.0

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1.0

LLC ICE-GB

p(shall | {shall, will})LLC ICE-GB total

shall 110 40 150will 78 58 136total 188 98 286

May be input in a

2 x 2 chi-square test

- or you can check Wilson intervals

0.0

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1955 1960 1965 1970 1975 1980 1985 1990 1995



• Plotting modal shall/will over time (DCPSE)

• Small amounts of data / year



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1955 1960 1965 1970 1975 1980 1985 1990 1995

p(shall | {shall, will})• Small amounts

of data / year

• Confidence intervals identify the degree of certainty in our results



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1955 1960 1965 1970 1975 1980 1985 1990 1995




• Highly skewed p in some cases

– p = 0 or 1 (circled)



0.0

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1955 1960 1965 1970 1975 1980 1985 1990 1995




• We can now estimate an approximate downwards curve


Recap Recap • Whenever we look at change, we must ask ourselves two things:

What is the change relative to?– Is our observation higher or lower than we might expect

• In this case we ask • Does shall decrease relative to shall +will ?

How confident are we in our results?– Is the change big enough to be reproducible?

ConclusionsConclusions

• An observation is not the actual value – Repeating the experiment might get different results

• The basic idea of inferential statistics is – Predict range of future results if experiment was

repeated• ‘Significant’ = effect > 0 (e.g. 19 times out of 20)

• Based on the Binomial distribution– Approximated by Normal distribution – many uses

• Plotting confidence intervals• Use goodness of fit or single-sample z tests to compare

an observation with an expected baseline• Use 22 tests or independent-sample z tests to compare

two observed samples

ReferencesReferences

• Aarts, B., Close, J., and Wallis, S.A. 2013. Choices over time: methodological issues in investigating current change. Chapter 2 in Aarts, B. Close, J., Leech G., and Wallis, S.A. (eds.) The Verb Phrase in English. Cambridge University Press.

• Wallis, S.A. 2013. Binomial confidence intervals and contingency tests. Journal of Quantitative Linguistics 20:3, 178-208.

• Wilson, E.B. 1927. Probable inference, the law of succession, and statistical inference. Journal of the American Statistical Association 22: 209-212

• NOTE: Statistics papers, more explanation, spreadsheets etc. are published on corp.ling.stats blog: http://corplingstats.wordpress.com

ma in english linguistics experimental design and statistics sean wallis survey of english usage...

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