Plug in principle

In statistics, the plug-in principle is the method of estimation of functionals of a population distribution by evaluating the same functionals at the empirical distribution based on a sample.

For example, when estimating the population mean, this method uses the sample mean; to estimate the population median, it uses the sample median; to estimate the population regression line, it uses the sample regression line.

It is called a principle because it is too simple to be otherwise, it is just a guideline, not a theorem.