How to tell when efficacy will NOT translate into effectiveness

I aim to show that the failure of the California Class Size Reduction initiative highlights an important class of situations in the application of evidence to policy. There are some circumstances in which the implementation of a policy will be self-defeating. The introduction of the factor assumed to have causal efficacy into the target population can lead to changes in the conditions of the target population that amount to interfering factors. Because these interfering factors are a direct result of the policy implementation they should be relatively easy to predict, and so part of the tricky issue of judging where evidence is relevant should in some circumstances be relatively straightforward. The failure of the California Class Size Reduction initiative also shows how important it is to identify the correct causal factor. The more accurate the attribution of causality, the less susceptible it will be to interfering factors and breaks in the causal chain.