Topic rankings and grades

In my history class I ask students to rank the topics a class covers three times: on the first day, at the midterm, and on the last day of class. I wanted to see if the rate of change for students was correlated at all with their course grades. For this exercise I did not look at the first ranking, as 23 of the 40 who were there chose the option "I don't know enough yet."

My spring 2012 class was on U.S. history from 1945 to the present, and I asked the class to rank the seven decades we covered ('40s through the '00s). A total of 27 students ranked all seven decades on both the midterm and at the end of class, and I divided them into three groups:

1. Minor changes (from 0 to 3 topic rankings changed)
   The 7 who were in this group had a GPA of 3.14.
2. Moderage changes (4 or 5 topic rankings changed)
   The 11 who were in this group had a GPA of 2.55.
3. Major changes (6 or all 7 topic rankings changed)
   The 9 who were in this group had a GPA of 2.11.

So what do my numbers mean? Maybe the students who already know their history can spend their class time focused on doing well, whereas the others are figuring out what they feel is important and not as concentrated on assignments. This question I am very interested in examining again.