Define the pull functions
These quantities should be normally distributed with means of zero and widths of one.
For the moment, ignore the y's. Then, since
= 
- 
and 
= 
- , it follows that
Thus,
or
But,
Distinguishing again between well-measured and poorly measured variables,
Thus,
so the pull is
Similarly, for poorly measured variables (if the off-diagonal elements are small),
so
