Excel is a very powerful product, but particularly when using statistical functions, you need to take care.
One example is standard deviation, which gives information on the spread of the data. Excel allows you to do it for a sample of the data (STDEV.S) and for the entire population (STDEV.P).
As a rule of thumb, if you don’t understand the difference between STDEV.S and STDEV.P; what a sample or population is or even what a standard deviation is, then don’t use it!