How many subjects should i enroll?

[galkim]

I am planning a prospective study that examines whether adding an ultrasound machine to a general physician clinic reduces the amount of referrals and blood tests done by that same physician.
For several months I want to look at 4 different physicians, give 2 of them an ultrasound machine, and in the middle of the study period give the machines to the other 2 physicians. I would then compare each physician's amount of referrals with/without the machine.
1. Which statistical tests should I use? I was thinking about Mann-Whitney.
2. What is the amounts of subjects I need to enroll in order to get significant results?

 

[hlsmith]

Are you going to make any comparisons across physicians? If so, will you need to control for differences in patients per provider - issues with patient exchangeability?

Also, the biggest question is usually, what is the exact outcome and how big of an effect do you expect?