In a typical regression model the actual value of a variable called "dependent" is "explained" by a set of regressors or explanatory variables. This can be represented as follows:
ACTUAL = FITTED + ERROR
The FITTED part of the regression model is what the analyst can measure, and estimate. It is referred to as the systematic component of the regression model.
ERROR refers to the difference (a residual) between what is observed and what is measured. This is also referred to as the unsystematic component of the regression model. opulation consists of the total collection of observations that are of interest to the decision maker.