I have been using z-scores by subtracting the mean (and then dividing by the SD) of the sample, whereas I recently read they actually need to be the population mean and SD. Assuming you are transforming raw scores to z-scores in order to compare within-sample variables (various test scores of the same sample of subjects), does it really invalidate the comparison if you have been using sample rather than population statistics?
Also, is it ever realistic that one would know the population statistics? I always assumed this is more of a theoretical concept than an actual statistic. For instance, in order for the population mean of the IQ test to be set to 100, surely this didn't imply testing every single human on the planet and adjusting the raw-to-standardized tables to reflect the population average! Many thanks.