Valores

std error = std deviation / sqrt(n) <-- n is the size of the sample.

if u are using the population in its entirety - the std deviation can be used directly.

however if u are using a sample to estimate parameters about the population - since the size of the sample makes a difference - you need to use the std error.

in another part of the curriculum - you would come across another rule of thumb - that when n (size of the sample) > 30, you can use z-statistic. Otherwise u need to use the t-statistic.

CP