Never mind the table, look at the sample size!

This morning I was just thinking that it’s been a while since I’ve filed anything in the category marked bad statistics when I glanced at today’s copy of the Times Higher and found something that’s given me an excuse to rectify my lapse. Last week saw the publication of said organ’s new Student Experience Survey which ranks  British Universities in order of the responses given by students to questions about various aspects of the teaching, social life and so  on. I had a go at this table a few years ago, but they still keep trotting it out. Here are the main results, sorted in decreasing order:

University Score Resp.
1 University of East Anglia 84.8 119
2 University of Oxford 84.2 259
3 University of Sheffield 83.9 192
3 University of Cambridge 83.9 245
5 Loughborough University 82.8 102
6 University of Bath 82.7 159
7 University of Leeds 82.5 219
8 University of Dundee 82.4 103
9 York St John University 81.2 88
10 Lancaster University 81.1 100
11 University of Southampton 80.9 191
11 University of Birmingham 80.9 198
11 University of Nottingham 80.9 270
14 Cardiff University 80.8 113
14 Newcastle University 80.8 125
16 Durham University 80.3 188
17 University of Warwick 80.2 205
18 University of St Andrews 79.8 109
18 University of Glasgow 79.8 131
20 Queen’s University Belfast 79.2 101
21 University of Hull 79.1 106
22 University of Winchester 79 106
23 Northumbria University 78.9 100
23 University of Lincoln 78.9 103
23 University of Strathclyde 78.9 107
26 University of Surrey 78.8 102
26 University of Leicester 78.8 105
26 University of Exeter 78.8 130
29 University of Chester 78.7 102
30 Heriot-Watt University 78.6 101
31 Keele University 78.5 102
32 University of Kent 78.4 110
33 University of Reading 78.1 101
33 Bangor University 78.1 101
35 University of Huddersfield 78 104
36 University of Central Lancashire 77.9 121
37 Queen Mary, University of London 77.8 103
37 University of York 77.8 106
39 University of Edinburgh 77.7 170
40 University of Manchester 77.4 252
41 Imperial College London 77.3 148
42 Swansea University 77.1 103
43 Sheffield Hallam University 77 102
43 Teesside University 77 103
45 Brunel University 76.6 110
46 University of Portsmouth 76.4 107
47 University of Gloucestershire 76.3 53
47 Robert Gordon University 76.3 103
47 Aberystwyth University 76.3 104
50 University of Essex 76 103
50 University of Glamorgan 76 108
50 Plymouth University 76 112
53 University of Sunderland 75.9 100
54 Canterbury Christ Church University 75.8 102
55 De Montfort University 75.7 103
56 University of Bradford 75.5 52
56 University of Sussex 75.5 102
58 Nottingham Trent University 75.4 103
59 University of Roehampton 75.1 102
60 University of Ulster 75 101
60 Staffordshire University 75 102
62 Royal Veterinary College 74.8 50
62 Liverpool John Moores University 74.8 102
64 University of Bristol 74.7 137
65 University of Worcester 74.4 101
66 University of Derby 74.2 101
67 University College London 74.1 102
68 University of Aberdeen 73.9 105
69 University of the West of England 73.8 101
69 Coventry University 73.8 102
71 University of Hertfordshire 73.7 105
72 London School of Economics 73.5 51
73 Royal Holloway, University of London 73.4 104
74 University of Stirling 73.3 54
75 King’s College London 73.2 105
76 Bournemouth University 73.1 103
77 Southampton Solent University 72.7 102
78 Goldsmiths, University of London 72.5 52
78 Leeds Metropolitan University 72.5 106
80 Manchester Metropolitan University 72.2 104
81 University of Liverpool 72 104
82 Birmingham City University 71.8 101
83 Anglia Ruskin University 71.7 102
84 Glasgow Caledonian University 71.1 100
84 Kingston University 71.1 102
86 Aston University 71 52
86 University of Brighton 71 106
88 University of Wolverhampton 70.9 103
89 Oxford Brookes University 70.5 106
90 University of Salford 70.2 102
91 University of Cumbria 69.2 51
92 Napier University 68.8 101
93 University of Greenwich 68.5 102
94 University of Westminster 68.1 101
95 University of Bedfordshire 67.9 100
96 University of the Arts London 66 54
97 City University London 65.4 102
97 London Metropolitan University 65.4 103
97 The University of the West of Scotland 65.4 103
100 Middlesex University 65.1 104
101 University of East London 61.7 51
102 London South Bank University 61.2 50
Average scores 75.5 11459
YouthSight is the source of the data that have been used to compile the table of results for the Times Higher Education Student Experience Survey, and it retains the ownership of those data. Each higher education institution’s score has been indexed to give a percentage of the maximum score attainable. For each of the 21 attributes, students were given a seven-point scale and asked how strongly they agreed or disagreed with a number of statements based on their university experience.

My current employer, the University of Sussex, comes out right on the average (75.5)  and is consequently in the middle in this league table. However, let’s look at this in a bit more detail.  The number of students whose responses produced the score of 75.5 was just 102. That’s by no means the smallest sample in the survey, either. The University of Sussex has over 13,000 students. The score in this table is therefore obtained from less than 1% of the relevant student population. How representative can the results be, given that the sample is so incredibly small?

What is conspicuous by its absence from this table is any measure of the “margin-of-error” of the estimated score. What I mean by this is how much the sample score would change for Sussex if a different set of 102 students were involved. Unless every Sussex student scores exactly 75.5 then the score will vary from sample to sample. The smaller the sample, the larger the resulting uncertainty.

Given a survey of this type it should be quite straightforward to calculate the spread of scores from student to student within a sample from a given University in terms of the standard deviation, σ, as well as the mean score. Unfortunately, this survey does not include this information. However, lets suppose for the sake of argument that the standard deviation for Cardiff is quite small, say 10% of the mean value, i.e. 7.55. I imagine that it’s much larger than that, in fact, but this is just meant to be by way of an illustration.

If you have a sample size of  N then the standard error of the mean is going to be roughly (σ⁄√N) which, for Sussex, is about 0.75. Assuming everything has a normal distribution, this would mean that the “true” score for the full population of Sussex students has a 95% chance of being within two standard errors of the mean, i.e. between 74 and 77. This means Sussex could really be as high as 43rd place or as low as 67th, and that’s making very conservative assumptions about how much one student differs from another within each institution.

That example is just for illustration, and the figures may well be wrong, but my main gripe is that I don’t understand how these guys can get away with publishing results like this without listing the margin of error at all. Perhaps its because that would make it obvious how unreliable the rankings are? Whatever the reason we’d never get away with publishing results without errors in a serious scientific journal.

This sampling uncertainty almost certainly accounts for the big changes from year to year in these tables. For instance, the University of Lincoln is 23rd in this year’s table, but last year was way down in 66th place. Has something dramatic happened there to account for this meteoric rise? I doubt it. It’s more likely to be just a sampling fluctuation.

In fact I seriously doubt whether any of the scores in this table is significantly different from the mean score; the range from top to bottom is only 61 to 85 showing a considerable uniformity across all 102 institutions listed. What a statistically literate person should take from this table is that (a) it’s a complete waste of time and (b) wherever you go to University you’ll probably have a good experience!

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14 Responses to “Never mind the table, look at the sample size!”

  1. Daniel Mortlock Says:

    And you didn’t even get to the further issue of how these small number of responding students were selected from their respective cohorts . . .

    • Exactly. If, for instance, a large number of people were asked and a small number responded (which is the way these things are often done in my experience), then the sample is almost certainly non-representative. People are much more likely to respond to a survey if they have strong opinions.

      My university places considerable weight in tenure and promotion decisions on student surveys of teaching quality. Response rates for these surveys are sometimes quite low. The people in charge assure me that they know the samples are representative, but I’ve never managed to get a clear statement of what data this is based on.

      • telescoper Says:

        It’s easy. If the response is favourable, the sample must be representative. If not, it must be biased.

      • Michael Kenyon Says:

        That must be a good thing as haven’t a lot of universities historically closed ranks when students have complained/ given feedback? The students who do give feedback might well be either very positive or negative, but at least they will hopefully be acknowledged this time. It might not be a big sample size but it is a start.

        Having studied at a couple of places on the list I always felt a few lecturers regarded students as something of an inconvenience.

        I’m not into having to pay for education, but one good thing to have happened is they are now much more open to scrutiny.

        How you interact with people affects how much you get paid/promoted in other workplaces, so why not universities?

        Not surprised UEA is at the top, lovely city and campus.

  2. Peter, you say you don’t understand how these guys can get away with publishing results like these without the associated errors.

    What, to me, is even more unfathomable is why university managements can get away with beating departments over the heads with them. Every year we have to write a report explaining why we’ve dropped x places in the NSS results (and what we plan to do about it) or write a piece for dissemination to the rest of the university on what aspects of our ‘excellent practice’ was responsible for us improving our position by y places.

    I have been told that I am not allowed to use the argument that tables of data such as NSS and THE are statistically meaningless.

  3. “Whatever the reason we’d never get away with publishing results without errors in a serious scientific journal.”

    One shouldn’t be able to, but sometimes things slip by. I remember once a histogram where the number of bins was appreciably larger than the number of objects.

  4. In this ApJ article, bad statistical practices in the astronomical community are criticized.

  5. John Peacock Says:

    Peter, you really should act in a more responsible manner and stop denigrating the efforts of those involved in producing league tables. If you carry on in this vein, you’ll end up concluding that the results from REF should carry error estimates. Clearly no right-thinking person could support such a suggestion.

    • telescoper Says:

      I’m fighting a losing battle over using error bars in our REF modelling, instead of taking our “critical friends” scores as Gospel, when trying to work out how selective to be.

  6. peter

    if we’re willing to assume the scores do not encode any information (ie they won’t change year-to-year because of any true improvements or declines in institutional quality), and THES claims the “methodology” is unchanged from 2012, then surely the variation in scores for institutes between 2012 and 2013 is a good proxy for the error in any particular score?

    i managed to track down the 2012 spreadsheet and a quick match of the institutes shows that the standard deviation of the differences in their scores between the two years is just 2.2 (with maximum changes of +5.1 and -6.1, so <3-sigma).

    which doesn't seem very likely, unless a lot of the score criteria are defaulted to specified values for each institute.

    ian

    • …there is something very odd going on – if you look at the number of respondents – there are a large spikes in the frequencies of institutions with ~100 or ~50 responses.

      are they culling responses to remove outliers in an attempt to suppress the variance?

      in fact if you just look at the standard deviation of the difference in scores between 2012/2013 for those institutions with > 120 responses the variation is even lower – just 1.2.

      i’ve mailed you the 2012 spreadsheet in case you want to play.

    • here’s some description of the “methodology” used for this polling:

      http://www.youthsight.com/media-centre/announcements/congratulations-to-uea-top-dogs-in-this-years-student-experience-survey-fieldwork-once-again-by-youthsight/

      this may explain the spikes in the respondent numbers if they repeatedly polled or chased respondents until they had either >50 (to be included at all) or >100 (which they appear to view as a threshold).

      the use of a 7-point scale does make me wonder if the minimal variance is because the respondents choose the middle point for most responses.

    • telescoper Says:

      Not quite right because the sample sizes are different in the two years. Also I think the individual scores are integers so there might be a peculiar effect of that..

  7. Seb Oliver Says:

    I noticed the spikes in response numbers too. Assuming no correlation between sample size and quality of institution I guess it should be possible to model the 2013 results on their own as containing an error (which should reduce with sample size as root (n)) and an underlying population variance which wouldn’t. I’ll see if I get a chance to look at this. As an tangential point, this is how we estimate confusion noise in Herschel maps, which would mean if this analysis goes anywhere I’d be able to claim it as REF impact!

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