Statistical tests for independent groups: categorical data
BMJ 2012; 344 doi: https://doi.org/10.1136/bmj.e344 (Published 18 January 2012) Cite this as: BMJ 2012;344:e344
- Philip Sedgwick, senior lecturer in medical statistics
- 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
- p.sedgwick{at}sgul.ac.uk
Researchers carried out a randomised controlled trial to compare the effectiveness of cryotherapy with that of salicylic acid for treating plantar warts. Participants randomised to cryotherapy were treated with liquid nitrogen by a healthcare professional, with a maximum of four treatments, each two to three weeks apart. Participants randomised to 50% salicylic acid (Verrugon) treated themselves daily for a maximum of eight weeks. The primary outcome was complete clearance of plantar warts at 12 weeks.1
The percentage of participants with complete clearance of plantar warts at 12 weeks was slightly higher in the salicylic acid group (17/119 (14.3%) v 15/110 (13.6%)), although the difference was not statistically significant (P=0.89).
Which one of the following statistical tests would most likely have been used to compare the treatment groups with regard to the percentage of participants with complete clearance of plantar warts at 12 weeks?
a) The χ2 test
b) Fisher’s exact test
c) McNemar’s test
Answers
The χ2 test (answer a) would most likely have been used to compare the treatment groups with regard to the percentage of participants …
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