A statistic that is widely used to determine how well a regression
fits is the coefficient of determination (or multiple correlation
coefficient), 
. represents the fraction of variability in
y that can be explained by the variability in x. In other words,
explains how much of the variability in the y's can be
explained by the fact that they are related to x, i.e., how close
the points are to the line. The equation for is
where SSTotal is the total sums of squares of the data.
NOTE:
In the simple linear regression case, is simply the square of
the correlation coefficient.