Day 022 - 12 Sep 94 - Page 55
1 results the analysis was restricted to 846 cases, so they
2 excluded a number of cases, and 862 controls. In other
3 words, less than 80 per cent of patients actually
4 responded to the questionnaires.
5
6 Now, you wonder, what about the other 20 per cent? The
7 difficulty with these studies, as I say, is the
8 methodology and the number of people who actually do not
9 respond. With a 20 per cent drop-out rate in this way,
10 you do wonder what influence that 20 per cent would have
11 if their results were available for the overall data. If
12 I run a clinical trial and could not collect 20 per cent
13 of the data, my colleagues would not think very much of
14 that clinical trial I was running.
15
16 Q. I will move on from that one.
17
18 MR. JUSTICE BELL: Is there a line at which someone carrying
19 out an investigation like this, a percentage which is
20 thought to be necessary to give reliability or not? You
21 say 80 per cent is not; that seems reasonably high to me.
22 It is more than anyone ever votes in an election in this
23 country, for instance.
24 A. That is true.
25
26 Q. Is there an accepted level which is the water-shed for
27 reliability?
28 A. I think most people would like to see 90 per cent
29 responses. You like to see 10 per cent or less of
30 non-respondents. If you are not getting evidence from a
31 fifth of the population you are trying to examine, that 20
32 per cent, if their results were different, could have a
33 tremendous influence on the overall results of the
34 study.
35
36 So, one wants to try to reduce that as low as possible,
37 preferably to no more than 10 per cent. This is one of
38 the major problems with case-controlled studies, that
39 often the controls who actually participate are people who
40 are enthusiasts. They may be people who are interested in
41 diet, therefore, feel they would like to respond to the
42 questionnaire. They may have other factors which single
43 them out. Therefore, although they may ostensibly appear
44 of the same age, the same sex and so on, one just wonders
45 are there other factors which may be different about them
46 which may explain some of the results that one finds? It
47 is a real dilemma in these studies.
48
49 MR. MORRIS: Can we move on from that study? Can I back track,
50 you remember the nurses study?
51 A. Yes.
52
53 Q. I am sure you are very well aware of it. One point you
54 mentioned about the identification of breast cancer was
55 that the fat content, the levels in the whole population
56 of the study were very high. In fact, I believe that the
57 whole study, the medium fat consumption at the start of
58 that study was 39 per cent of total daily calories,
59 indicating that it was about -- the population all had
60 high fat content in their diet. Is that correct?
