For objective causal inference, design trumps analysis

By Donald B. Rubin. 2008

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For obtaining causal inferences that are objective, and therefore have the best chance of revealing scientific truths, carefully designed and executed randomized experiments are generally considered to be the gold standard. Observational studies, in contrast, are generally fraught with problems that compromise any claim for objectivity of the resulting causal inferences. The thesis here is that observational studies have to be carefully designed to approximate randomized experiments, in particular, without examining any final outcome data. Often a candidate data set will have to be rejected as inadequate because of lack of data on key covariates, or because of lack of overlap in the distributions of key covariates between treatment and control groups, often revealed by careful propensity score analyses. Sometimes the template for the approximating randomized experiment will have to be altered, and the use of principal stratification can be helpful in doing this. These issues are discussed and illustrated using the framework of potential outcomes to define causal effects, which greatly clarifies critical issues. 1. Randomized experiments versus observational studies. 1.1. Historical dichotomy between randomized and nonrandomized studies for causal effects. For may years, causal inference based on randomized experiments, as described, for example, in classic texts by Fisher (1935), Kempthorne (1952), Cochran and Cox (1950 )a ndCox (1958), was an entirely distinct endeavor than causal inference based on observational data sets, described, for example, in texts by Blalock (1964), Kenny (1979), Campbell and Stanley (1963), Cook and Campbell (1979), Rothman (1986), Lilienfeld and Lilienfeld (1976), Maddala (1977 )a ndCochran (1983). This began to change in the 1970’s when the use of potential outcomes, commonly used in the context of randomized experiments to define causal effects since Neyman (1923), was used to define causal effects in both randomized experiments and observational studies [Rubin (1974)]. This allowed the definition of assignment mechanisms [Rubin (1975)], with randomized experiments as special cases, thereby allowing

Subfields: Statistics and Probability

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