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Table 4 Assumptions for sample size required to measure the primary outcome “proportion of girls who sit for the grade 9 exam”

From: Effectiveness of a girls’ empowerment programme on early childbearing, marriage and school dropout among adolescent girls in rural Zambia: study protocol for a cluster randomized trial

Parameter

Assumed level

Comment

Proportion of girls who sit for the grade 9 exam in control arm

0.70

 

Effectiveness of combined intervention vs control

+26.5%

i.e. proportion completing in combined intervention arm assumed to be 0.886

Effectiveness (i.e. (1-RR) × 100 of economic intervention vs control

+15%

i.e. proportion completing in economic intervention arm assumed to be 0.805

Effectiveness of combined intervention vs economic intervention

+10%

The combined intervention will offer a [1 + (0.886/0.805)], i.e. 10% relative increase compared to the economic intervention alone.

Cluster size

28

See assumptions for the outcome “incidence of births before girls’ 18th birthday”

ICC

0.02

We have no information on the ICC for this outcome but have assumed it to be higher than for pregnancy

Z1

1.96

 

Power for comparison of economic intervention vs combined intervention

95%

We need 29 clusters in each of the intervention arms to have 95% power to detect the assumed difference

Power for comparison of economic intervention vs control

95%

We need 24 clusters in each of the arms to have 95% power to detect the assumed difference

Power for comparison of combined intervention vs control

>95%

We need 7 clusters in each arm to have 95% power to detect the assumed difference