Is the method conservative?
The method has been used in many studies, including randomised controlled trials. The US Food and Drug Administration (FDA) used to recommend LOCF, considering it to be conservative ((1), pp. 16–17). Conservative means bias, such that the treatment effect is underestimated. However, it turns out that the method can yield bias in both directions, and it may even yield bias if data are missing completely at random ((2), pp. 47–50). In 2010, the National Research Council's Expert Panel on the prevention and treatment of missing data in clinical trials, which had been convened at the request of the FDA, concluded that neither LOCF nor BOCF should be used to handle missing data, unless the assumptions that underlie them are scientifically justified ((3), p. 77).
The comprehensive Handbook of Missing Data Methodology (2015) states that 'As LOCF is neither valid under general assumptions nor based on statistical principles, it is not a sensible method, and should not be used'. Other recent books on missing data also recommend against using LOCF. (1), p.16), ((5), p. 11) ((6), p. 59). Finally, I quote Vickers & Altman ((7): 'This method (LOCF) is attractive because it is simple, but it has little else to recommend it.'