 Retrospective observational studies work by ruling out alternative hypotheses such as these ex post rather than controlling for them ex ante as RCTs do (Reiss 2015a). In an RCT, mismeasurement should not obtain because the protocol specifies measurement procedures for the outcome variables in great detail in advance. Selection bias should not obtain because patients are randomized into treatment groups. Once allocated to a group, they are prevented from obtaining another treatment elsewhere, and researchers make sure that patients comply with the treatment regime. But there are equivalent means to rule out these possibilities in observational settings. While it may well be the case that early stages of cancer cause a craving for cigarettes, this hypothesis cannot explain the protective effect that stopping smoking has. At the time of the smoking/lung cancer controversy in the mid-1950s, misdiagnosis was indeed a problem. However, it could be shown that in order to account for the observed rise in lung cancer incidence, the diagnostic error at autopsy among older people would have to have been an order of magnitude  higher than the diagnostic error among younger people (Gilliam 1955). Mismeasurement could therefore also be ruled out. Similar considerations helped to rule out other alternative hypotheses (Cornfield et al. 1959).