# Standard Error of the Estimate (3 of 3)

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One typically does not know the population parameters and therefore has
to estimate from a sample. The symbol s

_{est} is used for the estimate
of σ

_{est}. The relevant formulas are:

and

where r is the sample correlation and S

_{y} is
the sample standard deviation of Y. (Note that S

_{y
} has a capital rather than a small "S" so it is
computed with N in the denominator). The similarity between
the standard error of the estimate and the standard
deviation should be noted: The standard deviation is the
square root of the average squared deviation from the mean;
the standard error of the estimate is the square root of the
average squared deviation from the regression line. Both
statistics are measures of unexplained variation.

Next section: Partitioning the sums of squares