Intended for healthcare professionals

Endgames Statistical Question

Selection bias versus allocation bias

BMJ 2013; 346 doi: (Published 24 May 2013) Cite this as: BMJ 2013;346:f3345
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

The effectiveness of supported self management in reducing hospital readmissions and death in patients with chronic obstructive pulmonary disease was evaluated.1 Researchers performed a randomised controlled trial. The intervention consisted of training patients to detect and treat exacerbations promptly, with ongoing support for 12 months. Patients in the control group continued to be managed by their general practitioner, hospital based specialists, or both.

Participants were patients admitted to one of six hospitals in the west of Scotland with an acute exacerbation of chronic obstructive pulmonary disease. In total, 464 patients were recruited and allocated to the treatment group using stratified randomisation based on demographic and disease severity factors. The main outcome measures included time until first hospital readmission or death owing to chronic obstructive pulmonary disease. The researchers reported that supported self management had no effect on time to first hospital readmission or death with chronic obstructive pulmonary disease.

Which …

View Full Text

Log in

Log in through your institution


* For online subscription