I'm estimating sample size for a future study using the primary outcome (mean and SD) from a current study. The future study will use a randomized control design with two independent 'arms' (or groups): Breathing Training or Balance Training.
The current study uses Breathing Training only, for which I have primary outcome data. However, no study using the Balance Training has used the same primary outcome (and therefore no mean and SD data).
Can I use a secondary outcome to estimate sample size for this Balance Training group?
That will reduce your sample size to half no harm in that but you need to consider the study population should have the same inclusion and exclusion criteria as your current study is having other wise there will be bias in your results. For sample size in balance training just take a same number of subjects as your current is having for the breathing group the combination of the two will be your sufficient sample size.This is what I understood from your question if you have something else then explain and ask again.
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