3.5 Error in cohort studies
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A major challenge for all cohort studies is the feasibility of long-term follow-up in terms of cost and resources. It can be very time-consuming to regularly trace all cohort subjects.
A loss to follow-up is also a major source of bias. Cohort participants may die, migrate, change jobs and even refuse to participate in the study. Loss can also be due to outcome e.g people who develop the condition may not be willing to continue with the study.