Department of Statistics, UCLA

In this paper, the strengths and weakness of randomized field ex- periments are discussed. Although it seems to be common knowledge that random assignment balances experimental and control groups on all confounders, other features of randomized field experiments are somewhat less appreciated. These include the role of random assign- ment in statistical inference and representations of the mechanisms by which the treatment has its impact. Randomized experiments also have important limitations and are sub ject to the fidelity with which they are implemented. In the end, randomized field experiments are still the best way to estimate causal effects, but are a considerable distance from perfection.