Excel on Office Business Standard 365

Hi

I wonder if anyone knows why Excel wont stabilise numerical data my numbers keep changing and changing my final calculation. What I am doing is a simple Mean Absolute Deviation of grouped data not more than 84 numerical entries. This isn't that important I'm just baffled by Excel incapability to handle this spreadsheet calculation. Does anybody know how tell Excel to keep my input still; i don't mean read only, just to stop them changing around.

Many thanks 

Brian