Analysis of first coitus.

The analysis of age of coital debut is central to a description of sexual behavior. Age of coital debut data typically shares some characteristics that make analysis difficult. For one, age is reported in whole years and, furthermore, some subjects do not report debut. We aimed to find a regression model that fits this type of data well and gives simple and interpretable results. We studied age of debut in four cross-sectional surveys (1987-2002) for a total of 18,000 subjects from the adult Norwegian population. We compared a Cox-model with a linear parametric survival model. Survival methods are natural tools for analyzing age of coital debut. The debut ages did not follow the proportional hazard model well and an additive parametric survival model was the better regression model for the Norwegian data set. Furthermore, the additive model was easier to interpret. The analysis showed a substantial change in age of debut in the cohorts born 1927-1984, with a drop of 1 year for men and 2.3 years for women. Women in the oldest cohorts reported their debut 0.8 years later and in the youngest cohorts 0.5 years earlier than did men. A parametric survival model gives results that are easier to interpret and fits the Norwegian data better than the Cox-model.