Endgames Statistical Question

Odds and odds ratios

BMJ 2013; 347 doi: https://doi.org/10.1136/bmj.f5067 (Published 16 August 2013) Cite this as: BMJ 2013;347:f5067
  1. Philip Sedgwick, reader in medical statistics and medical education
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, London, UK
  1. p.sedgwick{at}sgul.ac.uk

Researchers evaluated the efficacy of intravitreous injections of bevacizumab for the treatment of neovascular age related macular degeneration. A prospective, double blind, multicentre, randomised controlled trial study design was used. The intervention was intravitreous bevacizumab 1.25 mg, given as three loading injections at six week intervals and followed by further treatment if needed (again at six week intervals). The control was standard treatment—photodynamic therapy, intravitreal injections of pegaptanib, or intravitreal injections of placebo. Study participants were 131 patients (mean age 81 years) with wet age related macular degeneration.1

The primary outcome measure was a gain of 15 letters or more of visual acuity at one year from baseline assessed with an ETDRS (Early Treatment Diabetic Retinopathy Study) visual acuity chart. Of 65 participants allocated to the intervention, 21 (32%) gained 15 letters or more of visual acuity from baseline compared with two (3%) of the control group (n=66). The odds of a gain of 15 letters or more of visual acuity at one year were 21/44 for the intervention group and 2/64 for the control group. The unadjusted odds ratio for the primary outcome when comparing the intervention with the control was 15.3 (95% confidence interval 3.4 to 68.5). When adjusted for age, sex, and baseline visual acuity, the odds ratio for the primary outcome when comparing the treatments …

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