Intended for healthcare professionals

Endgames Statistical Question

External and internal validity in clinical trials

BMJ 2012; 344 doi: (Published 16 February 2012) Cite this as: BMJ 2012;344:e1004
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}

Researchers investigated the efficacy of a probiotic drink containing Lactobacillus for the prevention of antibiotic associated diarrhoea in patients over 50 years. A randomised double blind placebo controlled trial study design was used. The intervention consisted of consumption of a probiotic drink twice a day during a course of antibiotics and for one week after the course finished. The placebo was a long life sterile milkshake. The primary outcome was occurrence of antibiotic associated diarrhoea.1

Participants were recruited from three London hospitals. A total of 1760 patients were assessed for eligibility, 1625 of whom were not recruited because they did not meet the inclusion criteria (n=1263), refused to participate (n=148), or could not be included for practical reasons (n=214). The remaining 135 patients were recruited to the trial and randomised to intervention (n=69) or placebo (n=66). In total, 12 patients receiving the intervention and 10 in the placebo group did not complete their treatment protocol because they were lost to follow-up, withdrew consent, or died during the study period. …

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