Universal prevention (aimed at a population that has no increased risk for developing a mental disorder, such as school programs or mass media campaigns) need very high numbers of people to show effect (sometimes known as the "power" problem). Approaches to overcome this are (1) focus on high-incidence groups (e.g. by targeting groups with high risk factors), (2) use multiple interventions to achieve greater, and thus more statistically valid, effects, (3) use cumulative meta-analyses of many trials, and (4) run very large trials.