Clinical Trial - sample size for statistically comparable data?

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Hi Everyone

In a nutshell - I am looking to undertake an 8 week clinical trial of an equine veterinary health product. I am wanting to analyse corticosteroid concentrations deposited in the keratin of mane hair at Day 1 and then Day 56. I was hoping that 20 horses would be a sufficient number. Of these 20 horses, 10 would have an active treatment, whilst the remaining 10 would have a placebo treatment.

A couple of questions.
1). Is 20 horses a sufficient number?

2). Is the ratio of active to placebo treatments suitable?

I could at a push increase the number of horses involved within the trial to 25 or 30 - if that ensures more comparable data. IF I had an extra 5 or 10 horses, would it be best to put equal numbers on the active and placebo treatments? OR is it best to put these extra horses on the active treatment only and keep just the original 10 horses on the placebo?

I very much hope that makes sense! I would be grateful for any advice :tup:

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