kasinkei

I'm very good in statistics, and as far as I know, you have to use standard error when need to calculate the variability of the sample mean. That is, when someone says that they take a sample of size n and then calculates a mean of the sample what is the probability the mean falls in some interval, you use standard error (divide by sqrt(n)). When you are asked the probability a single observation falls in an interval, just use standard deviation. Thus,

Question asks about sample mean => Use standard error
Question asks about single observation => Use standard deviation