Loss to follow-up was used to estimate bias in a longitudinal study: a new approach

Objectives: To examine bias arising from loss to follow-up due to lack of contact. Study Design and Setting: The 1973-1978 cohort of Australian Longitudinal Study on Women's Health was first surveyed in 1996 and followed up in 2000, 2003, 2006, and 2009. At the 2000 survey, 9,688 women responded (responders), 2,972 could not be contacted, of whom 1,515 responded subsequently (temporary no contact) and 1,457 did not (permanent no contact). Characteristics were compared for these groups at baseline and follow-up in 2003, 2006, or 2009. Relative risk ratios were used to estimate bias. Results: No-contacts were younger, more likely to live in cities, to be less educated and stressed about money than responders. No-contacts were more likely to be in de facto relationships, separated, divorced, or widowed, to have experienced partner violence and be smokers. Compared with temporary no contact, permanent no contact were less educated, less likely to be studying or employed. Despite differences in prevalence estimates, relative odds ratios were close to one and had confidence intervals that included one, indicating little effect of bias. Conclusion: Although various characteristics were related to loss to follow-up, the relative risks estimates did not indicate serious bias due to loss to follow-up in this cohort of young women.