Theoretical Question Operational Data What is Sample and what is population.

Hi all.

I'm working on operational data of clinical operations, attendance, performance and employee performance.

I'm trying to use statistical methods to make regressions, correlations and predictions but I don't know if I should use the mean value and variance for the "sample" I have as being the population's or if I should consider it to be just a sample.

Some analyses require me to use the population's variance and mean and I don't know if I should use the ones calculated from the data I have or if I should infer it from them.

I have 2 years data on clinic attendance and performance for several clinics and staff members.

Thanks for your help.