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      Loss to Follow-Up in Cohort Studies: How Much is Too Much?

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          Abstract

          Loss to follow-up is problematic in most cohort studies and often leads to bias. Although guidelines suggest acceptable follow-up rates, the authors are unaware of studies that test the validity of these recommendations. The objective of this study was to determine whether the recommended follow-up thresholds of 60-80% are associated with biased effects in cohort studies. A simulation study was conducted using 1000 computer replications of a cohort of 500 observations. The logistic regression model included a binary exposure and three confounders. Varied correlation structures of the data represented various levels of confounding. Differing levels of loss to follow-up were generated through three mechanisms: missing completely at random (MCAR), missing at random (MAR) and missing not at random (MNAR). The authors found no important bias with levels of loss that varied from 5 to 60% when loss to follow-up was related to MCAR or MAR mechanisms. However, when observations were lost to follow-up based on a MNAR mechanism, the authors found seriously biased estimates of the odds ratios with low levels of loss to follow-up. Loss to follow-up in cohort studies rarely occurs randomly. Therefore, when planning a cohort study, one should assume that loss to follow-up is MNAR and attempt to achieve the maximum follow-up rate possible.

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          Author and article information

          Journal
          European Journal of Epidemiology
          Eur J Epidemiol
          Springer Science and Business Media LLC
          0393-2990
          August 2003
          August 2003
          : 19
          : 8
          : 751-760
          Article
          10.1023/B:EJEP.0000036568.02655.f8
          15469032
          7de2065d-28f3-424f-9293-5abe70e2cf01
          © 2003
          History

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