Self-completion questionnaires are a basic and much-used tool of the market researcher. Their great attraction is that by avoiding the need to employ a force of trained interviewers they considerably reduce the costs of undertaking a survey. This remains true despite sharply rising postage charges incurred in transmitting questionnaires from the researcher to his informants and back again. In the past, however, the use of self-completion has in the main been confined to questionnaires which have been extremely undemanding of informants. However complex the survey design, and however intricate the analyses performed on the data collected, the questionnaires themselves have normally been very short and very simple. This is, in a sense, paradoxical since the longer the questionnaire the greater the cost saving brought by using self-completion.