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. 2016 Jan;27(1):91-7.
doi: 10.1097/EDE.0000000000000409.

Selection Bias Due to Loss to Follow Up in Cohort Studies

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Free PMC article

Selection Bias Due to Loss to Follow Up in Cohort Studies

Chanelle J Howe et al. Epidemiology. .
Free PMC article

Abstract

Selection bias due to loss to follow up represents a threat to the internal validity of estimates derived from cohort studies. Over the past 15 years, stratification-based techniques as well as methods such as inverse probability-of-censoring weighted estimation have been more prominently discussed and offered as a means to correct for selection bias. However, unlike correcting for confounding bias using inverse weighting, uptake of inverse probability-of-censoring weighted estimation as well as competing methods has been limited in the applied epidemiologic literature. To motivate greater use of inverse probability-of-censoring weighted estimation and competing methods, we use causal diagrams to describe the sources of selection bias in cohort studies employing a time-to-event framework when the quantity of interest is an absolute measure (e.g., absolute risk, survival function) or relative effect measure (e.g., risk difference, risk ratio). We highlight that whether a given estimate obtained from standard methods is potentially subject to selection bias depends on the causal diagram and the measure. We first broadly describe inverse probability-of-censoring weighted estimation and then give a simple example to demonstrate in detail how inverse probability-of-censoring weighted estimation mitigates selection bias and describe challenges to estimation. We then modify complex, real-world data from the University of North Carolina Center for AIDS Research HIV clinical cohort study and estimate the absolute and relative change in the occurrence of death with and without inverse probability-of-censoring weighted correction using the modified University of North Carolina data. We provide SAS code to aid with implementation of inverse probability-of-censoring weighted techniques.

Figures

Figure 1
Figure 1
Causal diagram depicting five scenarios for the effect of injection drug use (A), heavy alcohol use (L), CD4 cell count (Q), and education (Z), on lost to follow up (D) and time to death (T) in a cohort study where u indexes time in visits since study entry and denotes that A, L, Q, Z, and D can vary with time.
Figure 2
Figure 2
Causal diagram depicting the association between African American race and time to death in the unweighted (top) and weighted (bottom) data among 2,511 HIV-infected African American and Caucasian men and women with 25,319 total person-visits of follow-up where u indexes time in visits since study entry, UNC CFAR HIV clinical cohort, 1999–2012.
Figure 3
Figure 3
Proportion alive (left) and risk ratio for death comparing African Americans to Caucasians (right) by visit among 2,511 HIV-infected men and women with 25,319 total person-visits of follow-up, UNC CFAR HIV clinical cohort, 1999–2012. The solid curve (Crude) does not correct for selection bias while the dashed curve (Weighted) corrects for selection bias due to loss to follow up dependent on African American race and measured covariates including insurance status and a prior AIDS-defining illness diagnosis at the first clinic visit as well as CD4 cell count and HIV RNA level at the prior visit using inverse probability-of-censoring weights.

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