CDP, Destroyer, kaplan science, and achiever are all harder than the actual and are great to use up until the last week or few days before the actual. If you do cdp for 2 months, and then take a topscore, you will laugh at how easy topscore pat is. It's good to get used to the harder materials as long as they don't depress you, and then right before the actual dat you hit up the confidence boosters (topscore, sample ADA).