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"Loss to follow-up" usually refers to loss of contact with a patient so that there is no opportunity to assess outcomes. A large proportion of participants lost to follow-up can invalidate the findings from a clinical trial. In this example, it appears that the 10-year probability of survival is identical for both treatment groups, 75%. However, a large number of patients in Group A were lost to follow-up and their survival status at 10 years is unknown. If the proportion surviving in the lost-to-followup group is different from the group for which survival is known, the conclusion that the two treatments are equally beneficial is incorrect.

Some losses to follow-up are probably inevitable. Although this example is extreme, it is not uncommon to find large proportions of patients lost, or an imbalance in proportions lost to follow-up if methods of maintaining contact are not applied diligently and equally to all treatment groups. Intensive efforts to retain contact with patients in all treatment groups and to identify alternative approaches to measuring outcome (such as national vital status records) should be incorporated.

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