Mathematical models for sexually transmitted infections (STIs) are parameterised by empirical data on sexual behaviour (e.g. the number of partners over a given period) obtained from surveys. However, the time window for reporting sexual partnerships may vary between surveys and how data for different windows can be translated from one to another remains an open question. To highlight this issue, we compared the distributions of the number of sexual partners over one year and four weeks from the British National Surveys of Sexual Attitudes and Lifestyles. The results show that simple linear rescaling did not render the one-year and four-week partner distributions aligned. Parameterising STI models using survey-based sexual encounter rates without considering the implication of the reporting window used can lead to misleading results.