The standard error measures the variability of an estimator (or sample statistic) from sample to sample. There are two approaches to estimating standard error:
1. The bootstrap. With the bootstrap, you take repeated simulated samples (usually resamples from the observed data, of the same size as the original sample, taken with replacement), calculate the estimator for each resample, then find the standard deviation of all the resampled estimators.
2. By formula. If you know the standard deviation of the sample data, you can use it to calculate an estimated standard error. For example, the formula for the standard error of the mean is
where STDEV is the standard deviation of the sample, N is the sample size.