- AForty is too small a number for any conclusion whatever to be drawn from it
- BThe clinic should have asked its patients to return after eight weeks rather than after six
- CWho came back was settled by the patients themselves, and a patient's reason for returning — or for not troubling to return — is likely to be bound up with whether the complaint cleared, so the 40 need not resemble the 300
- DThe clinic did not record the age of each patient it treated
Step-by-Step Solution
The 260 who stayed away were not a random 260: whether a patient bothers to come back turns on how he has fared, so the group that reported is shaped by the very thing being measured. A figure drawn from a group selected in that way cannot be carried over to the whole. Forty would be perfectly usable had it been chosen at random, the follow-up interval and the patients' ages are side issues.
Answer: C) Who came back was settled by the patients themselves, and a patient's reason for returning — or for not troubling to return — is likely to be bound up with whether the complaint cleared, so the 40 need not resemble the 300