BMJ  2008;336:573 (15 March), doi:10.1136/bmj.39514.402535.80

Letters

Diabetes education

Selection bias in cluster trial

The first 150 words of the full text of this article appear below.

The large cluster randomised controlled trial by Davies et al suffers from recruitment bias as a result of poor allocation concealment,¹ which is crucial in both individually randomised trials and cluster trials. Allocation was not concealed from the people doing the recruiting, so there is a danger of recruitment bias. Indeed, the nature of the intervention—an educational package—would be likely to increase recruitment bias—a form of selection bias. This possibly occurred in this trial as more of the intervention practices recruited participants and they each recruited more participants than the control practices. Even if the numbers had been similar, we could not be sure that participants were similar in unknown characteristics.

This design flaw has been pointed out in the past,² and it can be dealt with by using someone who is blind to the allocation and study hypothesis to recruit participants.³ We can only, at best, treat data from . . . [Full text of this article]

¹ York Trials Unit, University of York, York YO10 5DD

CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?

Relevant Articles

Effectiveness of the diabetes education and self management for ongoing and newly diagnosed (DESMOND) programme for people with newly diagnosed type 2 diabetes: cluster randomised controlled trial

M J Davies, S Heller, T C Skinner, M J Campbell, M E Carey, S Cradock, H M Dallosso, H Daly, Y Doherty, S Eaton, C Fox, L Oliver, K Rantell, G Rayman, K Khunti on behalf of the Diabetes Education and Self Management for Ongoing and Newly Diagnosed Collaborative
BMJ 2008 336: 491-495. [Abstract] [Full Text] [PDF]

Contamination in trials: is cluster randomisation the answer?

David J Torgerson
BMJ 2001 322: 355-357. [Extract] [Full Text] [PDF]