I am doing a project on carpal tunnel syndrome. I am looking at electrophysiological and symptomatic data before surgical intervention and again afterwards to assess whether symptomatic relief correlated with electrophysiological data. For numbers my clinic takes about 3000 referrals to investigate CTS. From this about half have carpal tunnel, and about a third go on to surgery.
My understanding was I didn't need to calculate sample size as my sample is limited by time/resources. In my proposal I stated:
It is anticipated that we will assess around 100 patients initially. From this, those who have a diagnosis of CTS and are listed to surgery will be invited to participate in follow up. The number of patients seen will vary somewhat depending on time constraints and may increase with time permitting.
The feedback I was given stated:
Why? Have you done a power calculation to ascertain the numbers needed in order to make a statistically valid judgement?"