Using target population specification, effect size, and reach to estimate and compare the population impact of two PTSD preventive interventions. 
The population impact of a preventive intervention depends on two factors: what proportion of the full population at risk receives the intervention and how large a reduction in risk occurs among those who receive it.  We sought to illustrate how information from a cognitive behavioral psychotherapy (CBT) trial and stepped collaborative care (CC) trial could be used to estimate the population impact of two contrasting approaches to PTSD prevention.  We first specified trauma center target populations represented by participants in each trial.  Patient characteristics were compared, as were effect size and reach indices and population-level reductions in PTSD incidence.  The CBT trial demonstrated a larger effect size (50% PTSD prevention), but minimal reach (27/10,000), while the CC trial demonstrated a smaller effect size (7% PTSD prevention) but greater reach (1762/10,000).  Modeling of the population impact suggested that a 9.5-fold greater cumulative reduction in the incidence of PTSD would result from the dissemination of the CC broad reach prevention strategy.  A reciprocal relationship between effect size and reach was evident in these two trials.  By specifying a target population, effect size and reach could be combined to project the overall population impact of each PTSD prevention approach.