Survival analysis is a powerful tool for analysing the results of total joint replacement, but it has major drawbacks when the failure rates are very low. We have reviewed 35 recent survival analyses of joint replacements to assess the magnitude of these problems and make recommendations as to how they may be avoided.

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Objectives To determine whether a serious paracetamol overdose in the medical television drama Casualty altered the incidence and nature of general hospital presentations for deliberate self poisoning. Design Interrupted time series analysis of presentations for self poisoning at accident and emergency departments during three week periods before and after the broadcast. Questionnaire responses collected from self poisoning patients during the same periods. Setting 49 accident and emergency departments and psychiatric services in United Kingdom collected incidence data; 25 services collected questionnaire data. Subjects 4403 self poisoning patients; questionnaires completed for 1047. Main outcome measures Change in presentation rates for self poisoning in the three weeks after the broadcast compared with the three weeks before, use of paracetamol and other drugs for self poisoning, and the nature of overdoses in viewers of the broadcast compared with non-viewers. Results Presentations for self poisoning increased by 17% (95% confidence interval 7% to 28%) in the week after the broadcast and by 9% (0 to 19%) in the second week. Increases in paracetamol overdoses were more marked than increases in non-paracetamol overdoses. Thirty two patients who presented in the week after the broadcast and were interviewed had seen the episode-20% said that it had influenced their decision to take an overdose, and 17% said it had influenced their choice of drug. The use of paracetamol for overdose doubled among viewers of Casualty after the episode (rise of 106%; 28% to 232%). Conclusions Broadcast of popular television dramas depicting self poisoning may have a short term influence in terms of increases in hospital presentation for overdose and changes in the choice of drug taken. This raises serious questions about the advisability of the media portraying suicidal behaviour.

Survival analysis of joint replacement relies on the assumption that surgical procedures in patients lost to follow-up have the same chance of failing as those in patients who continue to be assessed. Our study questions that assumption. During the 16-year follow-up of 2268 patients who had received total hip replacements 142 (6%) were lost to follow-up. The cumulative loss at 15 years was 20%. At their last assessment, patients who subsequently failed to attend for follow-up had significantly worse pain, range of movement and opinion of their progress (p < 0.001) and significantly worse radiological features than a matched control group (p < 0.01). Patients lost to follow-up have a worse outcome than those who continue to be assessed. Consequently, a survival analysis that does not take into account such patients is likely to give falsely optimistic results. It is therefore essential that vigorous attempts are made to minimise loss to follow-up, and that the rate of such loss is quoted. The overall loss to follow-up disguises the magnitude of the problem, which is best quantified by a cumulative rate of follow-up. The reliability of a study can be assessed by a loss-to-follow-up quotient, calculated by the number of failures: the lower the quotient the more reliable the data. Ideally, the quotient should be less than 1.