I don't know the reasoning, but our school puchased Math by all Means - the Probability Unit by Marilyn Burns to supplement this hole. It is for grades 3 & 4, and our state mandated test for fourth graders has enough probability questions on it to justify teaching it to the kids earlier than fifth. I think there are some 10 minute math activities in 3rd grade to introduce probability, but we didn't feel like it was enough. The Marilyn Burns activities are wonderful, and fit right in with the Investigations style of teaching.
