  Evidence for Action (E4A), a signature program of the Robert Wood Johnson
Foundation, funds investigator-initiated research on the impacts of social
programs and policies on population health and health inequities. Across
thousands of letters of intent and full proposals E4A has received since 2015,
one of the most common methodological challenges faced by applicants is
selecting realistic effect sizes to inform power and sample size calculations.
E4A prioritizes health studies that are both (1) adequately powered to detect
effect sizes that may reasonably be expected for the given intervention and (2)
likely to achieve intervention effects sizes that, if demonstrated, correspond
to actionable evidence for population health stakeholders. However, little
guidance exists to inform the selection of effect sizes for population health
research proposals. We draw on examples of five rigorously evaluated population
health interventions. These examples illustrate considerations for selecting
realistic and actionable effect sizes as inputs to power and sample size
calculations for research proposals to study population health interventions.
We show that plausible effects sizes for population health inteventions may be
smaller than commonly cited guidelines suggest. Effect sizes achieved with
population health interventions depend on the characteristics of the
intervention, the target population, and the outcomes studied. Population
health impact depends on the proportion of the population receiving the
intervention. When adequately powered, even studies of interventions with small
effect sizes can offer valuable evidence to inform population health if such
interventions can be implemented broadly. Demonstrating the effectiveness of
such interventions, however, requires large sample sizes.
