In the linked systematic review (doi:10.1136/bmj.b2732), Comondore and colleagues assess the relation between profit status and the quality of care in nursing homes and conduct a meta-analysis of four quality measures.1 The association between profit status and the quality of health care has been controversial for decades. The controversy stems partly from theoretical ambiguity and partly from lack of definitive empirical evidence. In theory, not-for-profit healthcare providers may provide a higher quality of care because their mission might include quality and because they do not need to divert resources to shareholders and taxes. On the other hand, for-profit providers may feel greater pressure to compete on price and quality, and this may result in higher quality care that is also more efficient. Unfortunately, rigorous testing of these competing theories is limited because it is impractical to conduct randomised controlled trials of profit status. Methods to mimic randomisation in observational studies (for example, instrumental variables2) are …