The League of Small Samples

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. Today 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. Here are the main results, sorted in decreasing order:

1 Loughborough University 84.9 128
2 University of Cambridge, The 82.6 259
3 University of Oxford, The 82.6 197
4 University of Sheffield, The 82.3 196
5 University of East Anglia, The 82.1 122
6 University of Wales, Aberystwyth 82.1 97
7 University of Leeds, The 81.9 185
8 University of Dundee, The 80.8 75
9 University of Southampton, The 80.6 164
10 University of Glasgow, The 80.6 136
11 University of Exeter, The 80.3 160
12 University of Durham 80.3 189
13 University of Leicester, The 79.9 151
14 University of St Andrews, The 79.9 104
15 University of Essex, The 79.5 65
16 University of Warwick, The 79.5 190
17 Cardiff University 79.4 180
18 University of Central Lancashire, The 79.3 88
19 University of Nottingham, The 79.2 233
20 University of Newcastle-upon-Tyne, The 78.9 145
21 University of Bath, The 78.7 142
22 University of Wales, Bangor 78.7 43
23 University of Edinburgh, The 78.1 190
24 University of Birmingham, The 78.0 179
25 University of Surrey, The 77.8 100
26 University of Sussex, The 77.6 49
27 University of Lancaster, The 77.6 123
28 University of Stirling, The 77.6 44
29 University of Wales, Swansea 77.5 61
30 University of Kent at Canterbury, The 77.3 116
30 University of Teesside, The 77.3 127
32 University of Hull, The 77.2 87
33 Robert Gordon University, The 77.2 57
34 University of Lincoln, The 77.0 121
35 Nottingham Trent University, The 76.9 192
36 University College Falmouth 76.8 40
37 University of Gloucestershire 76.8 74
38 University of Liverpool, The 76.7 89
39 University of Keele, The 76.5 57
40 University of Northumbria at Newcastle, The 76.4 149
41 University of Plymouth, The 76.3 190
41 University of Reading, The 76.3 117
43 Queen’s University of Belfast, The 76.0 149
44 University of Aberdeen, The 75.9 84
45 University of Strathclyde, The 75.7 72
46 Staffordshire University 75.6 85
47 University of York, The 75.6 121
48 St George’s Medical School 75.4 33
49 Southampton Solent University 75.2 34
50 University of Portsmouth, The 75.2 141
51 Queen Mary, University of London 75.2 104
52 University of Manchester 75.1 221
53 Aston University 75.0 66
54 University of Derby 75.0 33
55 University College London 74.8 114
56 Sheffield Hallam University 74.8 159
57 Glasgow Caledonian University 74.6 72
58 King’s College London 74.6 101
59 Brunel University 74.4 64
60 Heriot-Watt University 74.1 35
61 Imperial College of Science, Technology & Medicine 73.9 111
62 De Montfort University 73.6 83
63 Bath Spa University 73.4 64
64 Bournemouth University 73.3 128
65 University of the West of England, Bristol 73.3 207
66 Leeds Metropolitan University 73.1 143
67 University of Chester 72.5 61
68 University of Bristol, The 72.3 145
69 Royal Holloway, University of London 72.1 59
70 Canterbury Christ Church University 71.8 78
71 University of Huddersfield, The 71.8 97
72 York St John University College 71.8 31
72 University of Wales Institute, Cardiff 71.8 41
74 University of Glamorgan 71.6 84
75 University of Salford, The 71.2 58
76 Roehampton University 71.1 47
77 Manchester Metropolitan University, The 71.1 131
78 University of Northampton 70.8 42
79 University of Sunderland, The 70.8 61
80 Kingston University 70.7 121
81 University of Bradford, The 70.6 33
82 Oxford Brookes University 70.5 99
83 University of Ulster 70.3 61
84 Coventry University 69.9 82
85 University of Brighton, The 69.4 106
86 University of Hertfordshire 68.9 138
87 University of Bedfordshire 68.6 44
88 Queen Margaret University, Edinburgh 68.5 35
89 London School of Economics and Political Science 68.4 73
90 Royal Veterinary College, The 68.2 43
91 Anglia Ruskin University 68.1 71
92 Birmingham City University 67.7 109
93 University of Wolverhampton, The 67.5 72
94 Liverpool John Moores University 67.2 103
95 Goldsmiths College 66.9 42
96 Napier University 65.5 63
97 London South Bank University 64.9 44
98 City University 64.6 44
99 University of Greenwich, The 63.9 67
100 University of the Arts London 62.8 40
101 Middlesex University 61.4 51
102 University of Westminster, The 60.4 76
103 London Metropolitan University 55.2 37
104 University of East London, The 54.2 41
10465

The maximum overall score is 100 and the figure in the rightmost column is the number of students from that particular University that contributed to the survey. The total number of students involved is shown at the bottom, i.e. 10465.

My current employer, Cardiff University, comes out pretty well (17th) in this league table, but some do surprisingly poorly such as Imperial which is 61st. No doubt University spin doctors around the country will be working themselves into a frenzy trying how best to present their showing in the list, but before they get too carried away I want to dampen their enthusiasm.

Let’s take Cardiff as an example. The number of students whose responses produced the score of 79.4 was just 180. That’s by no means the smallest sample in the survey, either. Cardiff University has approximately 20,000 undergraduates. 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 Cardiff if a different set of 180 students were involved. Unless every Cardiff student gives Cardiff exactly 79.4 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. 7.94. 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 Cardiff, is about 0.6. Assuming everything has a normal distribution, this would mean that the “true” score for the full population of Cardiff students has a 95% chance of being within two standard errors of the mean, i.e. between 78.2 and 80.6. This means Cardiff could really be as high as 9th place or as low as 23rd, 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.

Still, at least there’s been one improvement since last year: the 2009 results gave every score to two decimal places! My A-level physics teacher would have torn strips off me if I’d done that!

Precision, you see, is not the same as accuracy….

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8 Responses to “The League of Small Samples”

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

    I’m sure one could find a few examples of such papers. IIRC there was one which binned some data but the number of bins was much larger than the number of data points.

  2. These results have as much impact as a table entitled “Universities most proficient in persuading students to fill out b******t questionnaires”

  3. -Sadly it would seem my peers have little else to do….

  4. Bryn Jones Says:

    All the same, the results do conform to my idea about how university league tables are compiled: obtain some statistics about universities, produce some index using some arbitrary combination of the data, rank the universities, then reject the index if Oxford and Cambridge do not come within the top few and find some new way of combining the data to form an index, then continue until Oxbridge finally comes top.

  5. telescoper Says:

    Bryn,

    Oxbridge isn’t top of this one. The best university in the country is actually Loughborough (pronounced “Lowbrow”).

    Peter

  6. Thanks for the explanation. Every year the University I work at bust a gut to try to get students to fill out the national student survey, on the assumption that students they’ve badgered into completing it will give a positive impression of the place.

    I understand the desire to know what students’ opinons of institutions are, but the tiny samples are pretty meaningless. Yet the effort and resources poured into it continue.

    • telescoper Says:

      Maybe in a few years time only a handful of students will be going to University, so such surveys will be more representative.

      The National Student Survey (NSS) is a different thing, actually. The one I was talking about was run by the Times Higher itself. The NSS attempts to survey all final-year undergraduates at each institution, but since it is voluntary there is a self-selection of respondents which probably introduces some sort of bias.

      I’ve got nothing against surveys of student opinion, or anything else for that matter, but it really annoys me when they are published without any discussion of their reliability. If such things are going to be published they should be accompanied by a clear warning about their reliability. Unfortunately, politicians, civil servants and, especially, University administrators are largely incapable of understanding arguments involving statistics so companies continue to peddle this twaddle and people lap it up.

  7. “Oxbridge isn’t top of this one. The best university in the country is actually Loughborough (pronounced “Lowbrow”).”

    A: Did you attend a high-brow university?

    B: No, Loughborough.

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