Models of the transmission of AIDS and of the spread of the epidemic use variables that describe sexual behaviour, e.g. the number of sexual partners, and coital frequency. It is, therefore, useful to collect information on these variables and to assess its validity. In this paper, we focus on data relating to coital frequency, given in the Survey on Sexual Behaviour in France (ACSF) undertaken in 1992. Reported coital frequency during the past four weeks is similar for men and for women (8.6 and 7.1 respectively). It decreases with age (after the age of 25) and duration of the union, falling from 13 per month during the union's first year, to less than 8 per month after 15 years. These results confirm those from earlier surveys, such as that by Simon in 1970. Frequency over the last four weeks is compared with habitual frequency, within single partnerhips. The correlation is quite strong, and shows that the two questions hardly differ in the view of respondents. Reported frequency can also be correlated with duration since last intercourse. The reciprocal of frequency provides an estimate of the interval between two acts of intercourse (for each individual). This will be a closed interval, whereas the time elapsed since last intercourse is an open interval. The conditions which make these two measures comparable are discussed. Assuming that an individual is bound to engage in intercourse with roughly the same probability every day, it is shown that the mathematical expectations of the lengths of both types of interval are the same. The survey data fit this model so perfectly that it may be concluded that both questions have received consistent answers. The further assumption that the daily probability of intercourse is lognormally distributed between individuals, makes it possible to estimate the entire distribution of intervals. However, it is possible that all the data are marred by the same bias : the tendency to standardize reported behaviour. This would lead to an overestimate of the strength of the correlation, and perhaps of the habitual frequency of sexual relations.
