Given a data set, how do you know whether to calculate sigma or s...
When given a data set, one would have to determine if it represented the population or if it was a sample taken from the population.
If the data are a population, then is calculated. If the data are a sample, then s is calculated.
S is the sample standard deviation, it is calculated when the observations represent a sample of the population. sigma is the population standard deviation, it is calculated when de observations are the whole population.
it would be 6xy and -16xy.