Does Attrition during Follow-Up of a Population Cohort Study Inevitably Lead to Biased Estimates of Health Status?

Abstract

Attrition is a potential source of bias in cohort studies. Although attrition may be inevitable in cohort studies of older people, there is little empirical evidence as to whether bias due to such attrition is also inevitable. Anonymised primary care data, routinely collected in clinical practice and independent of any cohort research study, represents an ideal unselected comparison dataset with which to compare primary care data from consenting responders to a cohort study. Our objective was to use this method as a novel means to assess if (i) responders at follow-up stages in a cohort study remain representative of responders at baseline and (ii) attrition biases estimates of longitudinal associations. We compared primary care consultation morbidities and prescription prevalences among circa 32,000 patients aged 50+ who contribute to an anonymised general practice database (Consultations in Primary Care Archive (CiPCA)) with those from patients aged 50+ in the North Staffordshire Osteoarthritis Project (NorStOP) cohort, United Kingdom (2002–2008; n = 16,159). 8,197 (51%) persons responded to the NorStOP baseline survey and consented to medical record review. 5,121 and 3,311 responded at 3- and 6-year follow-ups. Differences in consulting prevalence of non-musculoskeletal morbidities between NorStOP responders and CiPCA comparison population did not increase over the two follow-up points except for ischaemic heart disease. Differences observed at baseline for osteoarthritis-related consultations were generally unchanged at the two follow-ups (standardised prevalence ratios for osteoarthritis (1.09–1.13) and joint pain (1.12–1.23)). Age and gender adjusted associations between baseline consultation for chronic morbidity and future new osteoarthritis and related consultations were similar in CiPCA (adjusted Hazard Ratio: 1.40; 95% Confidence Interval: 1.34,1.47) and NorStOP 6-year responders (1.32; 1.15,1.51). There was little evidence that responders at follow-ups represented any further selection bias to that present at baseline. Attrition in cohort studies of older people does not inevitably indicate bias.

Weak Grip Strength and Cognition Predict Functional Limitation in Older Europeans

“…First, longitudinal data are often incomplete or unbalanced because of loss to follow‐up. Previous studies reported that attrition in the longitudinal survey was more complicated than often assumed, and attrition may not inevitably indicate bias and limit the generalizability of longitudinal comparisons . We used mixed‐effect linear regression models, which provided appropriate techniques for managing such a challenge.…”

“…Previous studies reported that attrition in the longitudinal survey was more complicated than often assumed, and attrition may not inevitably indicate bias and limit the generalizability of longitudinal comparisons. 46,47 We used mixed-effect linear regression models, which provided appropriate techniques for managing such a challenge. Mixed-effect linear regression models can account for unbalanced or incomplete data under the assumption that observations are missing at random.…”

Attrition in a 30-year follow-up of a perinatal birth risk cohort: factors change with age

“…In order to interpret follow-up results, bias must be analyzed, corrected, and taken into account in clinical interpretation (Guyatt, 2009). However, even high proportions of LTFU do not necessarily cause uncontrollable bias for endpoint measures of a trial (Lacey, Jordan & Croft, 2013; Gustavson et al, 2012), e.g., a high proportion of LTFU makes it unreliable to calculate prevalences but cause—consequence analyses are still possible (Martikainen et al, 2007). In follow-up studies and trials that last for decades, it is next to impossible to achieve attrition less than 20–40%.…”

Optimal primary care management of clinical osteoarthritis and joint pain in older people: a mixed-methods programme of systematic reviews, observational and qualitative studies, and randomised controlled trials

“…A recent study of the potential effects of attrition in the NorStOP cohorts confirmed that there was little evidence that responders at follow-up points represented any further selection bias to that present at baseline. 79 …”