Sampling strategies to measure the prevalence of common recurrent infections in longitudinal studies

Table 3 Multiplication factors for the required sample sizes

	52 visits	20 visits	12 visits	6 visits	4 visits
Model scenario 1 (LS)
7 day point prevalence recall	1.0	1.4	1.9	3.3	4.8
3 day point prevalence recall	1.0	1.6	2.4	4.2	6.3
7 day period prevalence recall	1.0	1.4	1.9	3.2	4.6
Incidence during 7 day recall	1.0	1.6	2.2	3.7	5.2
Model scenario 2 (LL)
7 day point prevalence recall	1.0	1.1	1.4	2.2	2.9
3 day point prevalence recall	1.0	1.2	1.6	2.4	3.4
7 day period prevalence recall	1.0	1.1	1.4	2.2	2.9
Incidence during 7 day recall	1.0	1.5	2.1	3.7	5.1
Model scenario 3 (HS)
7 day point prevalence recall	1.0	1.1	1.3	1.7	2.1
3 day point prevalence recall	1.0	1.2	1.4	2.0	2.6
7 day period prevalence recall	1.0	1.1	1.3	1.8	2.3
Incidence during 7 day recall	1.0	1.3	1.7	2.5	3.3
Model 4 scenario (HL)
7 day point prevalence recall	1.0	1.1	1.2	1.5	1.8
3 day point prevalence recall	1.0	1.1	1.3	1.6	2.0
7 day period prevalence recall	1.0	1.1	1.2	1.5	1.8
Incidence during 7 day recall	1.0	1.4	1.8	2.7	3.7

Multiplication factors for the required sample size to achieve the same statistical power as a study using weekly disease sampling over one year (52 visits). Values are given for 4 different surveillance strategies (in addition to the baseline strategy) to measure disease prevalence applied to the four different model scenarios. For comparison, these values are also calculated for using incidence instead of prevalence. Incidence was defined as the occurrence of any new episode during the 7 day recall period with a gap of at least 2 days between two new episodes. The assumptions on recall error were the same as for prevalence measures (but these did not impact on the model results, see sensitivity analysis).

ISSN: 1742-7622