is about estimate of standard error of the mean Alejandra Gonzalez-Beltran https://fevir.net/resources/CodeSystem/27270#STATO:0000037 Brian S. Alper, Harold Lehmann, Muhammad Afzal, Xing Song, Kenneth Wilkins, Joanne Dehnbostel Orlaith Burke Philippe Rocca-Serra SEM SEM The standard error of the mean (SEM) is data item denoting the standard deviation of the sample-mean's estimate of a population mean. It is calculated by dividing the sample standard deviation (i.e., the sample-based estimate of the standard deviation of the population) by the square root of n , the size (number of observations) of the sample. A measure of dispersion applied to means across hypothetical repeated random samples. adapted from wikipedia (https://en.wikipedia.org/wiki/Standard_error) http://stats.stackexchange.com/questions/50623/r-calculating-mean-and-standard-error-of-mean-for-factors-with-lm-vs-direct scipy.stats.sem(a, axis=0, ddof=1) http://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.sem.html#scipy.stats.sem source: https://github.com/scipy/scipy/blob/v0.15.1/scipy/stats/stats.py#L1928 statistic sample mean standard error of estimate ready for release