I am writing a research proposal to determine the clinical efficacy of 3 different types of orthotic devices and 1 sham device for the treatment of plantar fasciitis but am having trouble narrowing down the sample size required. Farrokhyar et al. 2013 suggested 63 per arm is required for a 2 arm trial so do I simply multiply by 4. I have also seen a suggestion that the sham arm needs less in but cannot seem to find this suggestion again. I believe it was suggested that the sham arm would only need about 47 due to anticipated less changes. I have also seen a suggestion that if more arms are used the number within each group has to increase. Just seem to be getting myself more and more confused. Any suggestions?
Hi Tracy,
For the appropriate answer, you need to provide some more info about your methods: i) Are you going to test initial / acute effect of those orthoses, or you are going to apply them in a length of time. ii) Do you plan to apply three different orthoses (plus sham) on same group, or will you create 4 different groups for each orthoses and sham?
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