Establishing the evidence base for psychiatric services: Estimating the impact on the population

The impact of evidence-based interventions for individual patients is clear, but a broader approach is needed. The authors describe a six-step method for calculating the population impact of introducing a new evidence-based intervention. The steps involve identifying the prevalence of a condition, the proportion of persons in the larger population eligible for the intervention, and the baseline risk that the intervention is meant to address. Obtaining useful results from this mathematical calculation depends on having accurate and robust data about the prevalence of a condition, the size of the targeted population, and the effectiveness of the intervention. Results for different interventions for the same condition and target population can then be compared.