When is not known we must come up with other statistical tools to estimate or test the population mean. Typically, we have been substituting the sample standard deviation sfor the standard deviation of x, sn, for . When the standard deviation of a statistic, like x, is estimated from the data, the result is called the standard errorof the statistic. ex. SEsnxRecall, that our basis for inference so far has revolved around zxnwhere znormal(0,1) if is known.