Consistency across studies provides a compelling basis for causal inferences in
epidemiologic research. Examining the same hypothesized relationship between levels of
vitamin D and pulmonary function in diverse populations will be important. Seeing a similar
relationship across different studies involving different sample populations controlling for
potential confounders in various ways, strengthens the case for causality. Observing a dose-
response relationship, as suggested in the current study by Black and colleagues, 1 …