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Analysis And Comment Research methods

Reporting attrition in randomised controlled trials

BMJ 2006; 332 doi: (Published 20 April 2006) Cite this as: BMJ 2006;332:969
  1. Jo C Dumville, research fellow (,
  2. David J Torgerson, director1,
  3. Catherine E Hewitt, PhD student1
  1. 1 York Trials Unit, University of York, York YO10 5DD
  1. Correspondence to: J C Dumville
  • Accepted 15 February 2006

Loss to follow-up can greatly affect the strength of a trial's findings. But most reports do not give readers enough information for them to be able to understand the potential effects

The main evaluative strength of randomised controlled trials is that each group is generally balanced in all characteristics, with any imbalance occurring by chance. However, during many trials participants are lost to follow-up. Such attrition prevents a full intention to treat analysis being carried out and can introduce bias.1 2 Attrition can also occur when participants have missing data at one or more points. We argue that researchers need to be more explicit about loss to follow-up, especially if rates are high.

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

Reporting of attrition

In a review of trials published in four general medical journals in 2002, 54% (71) of the 132 trials had some loss to follow-up for the main analysis.4 Among these trials the …

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