Surveillance of sexually transmitted infections in primary care

Table 1

Number of people required to detect differences in prevalence*

	Prevalence (%)
Difference between populations (%)	2	5	10	20
*Calculations assumed a random sample, 5% two sided significance level.
−3	–	653	1421	2695
−2	482	1604	3312	6138
−1	2514	6943	13 694	24 840
+1	4023	8357	14 950	25 782
+2	1239	2311	3940	6609
+3	653	1125	1840	3009

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