Howlerkarma, my lack of enthusiasm for AP Stats is related to the mastery issue.
I think statistics is a beautifully logical and consistent subject as well as an important one. But the right way to understand it is to start by learning probability, then learn the random variable concept, and then think of statistics as functions of random variables. If you do this everything makes a great deal of sense.
In my daughter's AP Statistics class they went in the opposite order. They spent the first few months talking about statistics and then did probability later in the year. When you do it this way, it seems like it becomes an exercise in learning that there are a large number of formulas you'd apply in different situations, but doesn't form a coherent whole.
In our department we teach statistics using Degroot and Schervish which is good, but I know it requires calculus and perhaps also some linear algebra. I don't know if there are AP-level textbooks that go in the more mathematically appealing order. (Like physics I also think statistics is better done using calculus.)
One other thing I would recommend if you are doing AP Stats is to also work with some statistical programming package. Our daughter's class did a lot of things with a graphing calculator which I thought was insane. No one really doing any statistical analysis would start by typing a dataset into a calculator, so I see no point in learning how to do that.
ElizabethN, I hope she likes it. For most 9-year-olds I'd start with Chapter 1 to build comfort. For some advanced and/or impatient kids Chapter 2 can will be a better place to start to let them know there will be things that are more novel.
