Generating and comparing summary statistics across a population and subset

I have a population that I am deriving lifespan from (n=41). I have calculated the mean, median, interquartile (1st and 3rd) ranges, min and max values for the total number of samples. I have then grouped all the data into month (n=12, being jan-dec) and separately year (n=14yrs) producing means for each time period and then generated an overall mean (in addition to median and interquartile ranges).

Can I compare the mean value calculated from all samples to those of generated for the average month and year??...

Reviewing the calcs that I have generated is not what I expect - I assumed with the increase in 'precision' that the median would decrease: all -> yr -> month.

What is the most appropriate way to generate the respective summary statistics so they are accurate.

Any assistance would be appreciated