Ergodicity is sufficient but not necessary for group-to-individual generalizability

Researchers commonly draw inferences from the group level to the individual and vice versa—that is, across levels. One of the empirical cornerstones of medicine is the clinical trial that tests the efficacy of a drug compared with placebo. If the intervention group outperforms the placebo group, the conclusion is that the drug should be prescribed for individuals with a given disorder. When are such inferences across levels defensible? In their recent paper in PNAS, Fisher et al. (1) state that “statistical findings at the interindividual (group) level only generalize to the intraindividual (person) level if the processes in question are ergodic,” meaning that the effects of … [↵][1]1To whom correspondence should be addressed. Email: janne.adolf{at}kuleuven.be. [1]: #xref-corresp-1-1