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Endgames Statistical Question

Randomised controlled trials: understanding power

BMJ 2015; 350 doi: (Published 18 June 2015) Cite this as: BMJ 2015;350:h3229
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Institute for Medical and Biomedical Education, St George’s, University of London, London, UK
  1. Correspondence to: P Sedgwick p.sedgwick{at}

The effects of manual lymph drainage on the development of lymphoedema related to breast cancer were investigated using a randomised controlled trial.1 The intervention was a six months’ treatment programme consisting of guidelines about prevention of lymphoedema, exercise therapy, and manual lymph drainage. Control treatment consisted of the same programme as the intervention but without the guidelines about manual lymph drainage. Participants were consecutive patients with breast cancer and unilateral axillary lymph node dissection. The length of follow-up was 12 months after surgery. The setting was hospitals in Belgium.

The outcome measures included the cumulative incidence of arm lymphoedema by follow-up. Arm lymphoedema was defined as an increase in arm volume of 200 mL or more in the value before surgery. The sample size was based on having 80% power to detect a difference between treatment groups of 20% in the cumulative incidence of arm lymphoedema, assuming a cumulative incidence of 30% for the control group at follow-up. The sample size calculation assumed a two sided hypothesis test and critical level of significance of 0.05 (5%). In total, 146 patients were required. To account for an estimated dropout rate of 10%, the required sample size was adjusted to 160 patients. In total, 160 patients were recruited, with 79 allocated to the intervention, and 81 allocated to control.

Overall, 154 (96.3%) patients completed follow-up, with four patients in the intervention group and two in the control group lost to follow-up. At 12 months after surgery, the percentage of patients with arm lymphoedema was higher in the intervention group than in the control group, although the difference was not significant (24% (n=18) versus 19% (n=15); difference 5%, 95% confidence interval −8% to 18%; P=0.45). It was concluded that there was no evidence that manual lymph drainage in addition to guidelines and …

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