From Association to Causation in Observational Studies: The Role of Tests of Strongly Ignorable Treatment Assignment

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.