Now, if the retention volume of a solute at the outlet pressure (Po) is (Vr(o)) and the true retention volume is (Vr) measured at the mean column pressure (Pr), then
PrVr = PoVr(o)
Thus,
(24)
It is now necessary to derive an expression for (Pr)
and then the correction factor 
can be evaluated. The average
pressure in the column (Pr) is defined
as, 
.
Now it has been shown that
Thus,
