Hello,
I'm a student working with the Keithley 590 C-V Analyzer, and my testing set up is using a switching matrix and its adding about 5-20 pF in parasitic capacitance. I read the manual for the analyzer and recommended the calibration capacitor correction method. But when I apply the method the correction isn't working properly. I was wondering if there was anybody that could maybe help. I've attached the manual for reference. The calibration method is on page 247 of the pdf or 4-102 on the manual. 
Best,
Julian