Systematic component
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.