Causal Inference in Biomedical Research

Causation can be inferred by two distinct patterns of reasoning, each requiring a distinct experi-mental design. Common, non-statistical causal inference is associated with controlled experi-ments in basic biomedical research. Statistical inference is associated with Randomized Con-trolled Trials in clinical research. The main difference between the two patterns of inference hinges on the satisfaction of a comparability requirement, which is in turn dictated by the nature of the objects of study, namely homogeneous vs. heterogeneous populations of biological sys-tems. This distinction entails that the objection according to which randomized experiments fail to provide better evidence for causation because randomization cannot guarantee comparability is mistaken. As far as the validity of the statistical inference is concerned, randomization is not re-quired in order to ensure comparability, but rather to prevent systematic bias which may com-promise the accuracy of the intervention