Standard error of mean in a population is calculated by the formula:

Although the sampling distribution of the mean is approximately normally distributed, the sampling distributions of most other statistics are not. Therefore the population standard detion is estimated by the sample standard detion SD, and the SD is used in place of the population value in the calculation of the standard error; that is, the standard error in the population is estimated by, Ref: (2004). Chapter 4. Probability & Related Topics for Making Inferences About Data. In Dawson B, Trapp R.G. (Eds), Basic & Clinical Biostatistics, 4e.