Reprint of “Observational Studies” and Comments

William Cochran’s paper “Observational Studies” is reprinted followed by comments by current researchers in observational studies: Norman Breslow; Thomas Cook; David Cox & Nanny Wermuth; Stephen Fienberg; Joseph Gastwirth & Barry Graubard; Andrew Gelman; Ben Hansen & Adam Sales; Miguel Hernan; Jennifer Hill; Judea Pearl; Paul Rosenbaum; Donald Rubin; Herbert Smith; Mark van der Laan; Tyler VanderWeele; and Stephen West.