This can be done by drawing first a random sample of practices, and then, within these practices, drawing a random sample of individuals. Such two stage sampling works well, but there is some loss of statistical efficiency, especially if only a few units are selected at the first stage. Recruiting subjects Most people are willing to take part in medical surveys provided that they trust the investigators, just as patients will nearly always help their own doctors in their research. In population studies, however, there has usually been no previous contact. Local general practitioners, too, need to know what is going on.
Time given to preparatory public relations is always well spent.
Response must be made as easy as possible. If attendance at a centre is required, it is better to send everyone a provisional appointment than to expect them to reply to a letter asking whether they are willing to attend. Provision of transport may be welcomed. The level of response that is acceptable depends both on the study question and on the population in which the question is being asked. Problems arise because non-responders may be atypical.