This information is difficult to obtain through randomized trials. First, policy typically affects a large number of individuals indirectly and has relatively minor effects. Therefore, randomized trials for health policy research would require huge samples and randomization at the level of larger units—cities, counties, and states. Second, the relatively short window of opportunity to inform policy, often just a few months, leaves no time to conduct complex randomized studies. Third, the policy and health care environments change much more quickly than human biology. Data collected five years ago may already be obsolete, whereas data from clinical efficacy studies can be cumulative over many years. Thus we are lucky to find any current data to evaluate a policy proposal, whereas the frequent problem in clinical medicine is the large number of trials, some with conflicting findings. Systematic reviews and meta-analyses can sort out conflicting messages, but they are of no help if there are few data to begin with.