I am designing a study to examine the difference made by implementing a new treatment strategy for a medical condition. I have become a little stuck on some of the statistics.

I want to determine if the new strategy is effective.

I can assume that the population number is fixed as all patients that underwent the old strategy last year will be switched to the new one this year.

I have a figure for the total number of complications for last year and want to know how many fewer complications must occur this year for the difference to be statistically significant. (p<0.05).

Does anyone know what test I should use or how I calculate this?

Any help would be much appreciated.

Thank you.