Reporting attrition in randomised controlled trials

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Abstract
Effects of attrition Attrition can introduce bias if the characteristics of people lost to follow-up differ between the randomised groups. In terms of bias, this loss is important only if the differing characteristic is correlated with the trial's outcome measures. However, attrition is not a black and white issue—there is no specific level of loss to follow-up at which attrition related bias becomes acknowledged as a problem. Schulz and Grimes argue that loss to follow-up of 5% or lower is usually of little concern, whereas a loss of 20% or greater means that readers should be concerned about the possibility of bias; losses between 5% and 20% may still be a source of bias.3 For the purposes of this article we will not differentiate between loss to follow-up and missing data. We have also not considered exclusions by trial investigators. Although exclusion is justified in some cases,3 generally it is ill advised.1 2