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It is important to know who takes part in screening programmes because this will help to understand and interpret information about their outcomes. The English National Chlamydia Screening Programme (NCSP) was introduced in the first areas in September 2002.1 Riha et al2 make an important contribution to the literature about screening programmes in general and chlamydia screening in particular. They showed that women and men participating in the NCSP have higher levels of sexual risk behaviours and that they are about three times as likely to be infected with Chlamydia trachomatis (chlamydia) infection than their counterparts in the general population, who took part in the UK National Survey of Sexual Attitudes and Lifestyles 2000 (Natsal-2).
Riha et al's study2 shows the importance of defining the denominator population when describing the occurrence of chlamydia infection. Prevalence is the total number of all individuals who have a disease at a particular time divided by the population at risk of having the disease.3 Riha et al can report the …