Uses the univariate variance estimates computed from the samples of X and Y. This turns out to be the standardized first principal component. This option is not a good choice in a measurement systems application since the error variances are not likely to be proportional to the population variances.


Uses 1 as the variance ratio, which assumes that the error variances are the same. Using equal variances is equivalent to the nonstandardized first principal component line. Suppose that the scatterplot is scaled the same in the X and Y directions. When you show a normal density ellipse, you see that this line is the longest axis of the ellipse.


Uses a variance ratio of zero, which indicates that Y effectively has no variance.

