When I was studying for the PMP exam, a few years ago, I remember memorizing a group of formulas.  One of those was the "Variance of Activity."  At this point, don't remember if it was even referenced in the exam.  There were no direct questions asking "what is the formula for..."  On my exam, I remember having numerous questions resulting from schedule variance calculations and cost variance calculations.  To my surprise, I went searching for the Variance of Activity formula in the PMBOK (4th Edition) and I can't find it!  So as not to lead people astray when giving PMP study advice, I'm now researching each formula I was once told to memorize.  I'm very surprised PMI didn't save us a lot of trouble and list known formulas in the back of the PMBOK.