If treatment assignment is strongly ignorable, then adjustment for observed covariates is sufficient to produce consistent estimates of treatment effects in observational studies. A general approach to testing this critical assumption is developed and applied to a study of the effects of nuclear fallout on the risk of childhood leukemia. R.A. Fisher's advice on the interpretation of observational studies was “Make your theories elaborate”; formally, make causal theories sufficiently detailed that, under the theory, strongly ignorable assignment has testable consequences.