I tend to believe personality of the student has more bearing upon the success of the group than how I group them. The greastest success of groups are with the students that enjoy group work and will work at cooperation and know how to get along with people. I have found that if I group students that are a great distance in ability then the group is not as successful. The better student many times displays discontent in helping the group member that is not at the level the student should be. The better student wants to go on and build on his knowledge and feels like too much time may be spent helping members of the group.
Some students feel no challenge in working in a group where all they do is help everybody catch up.
These are only my observations of my students as we have worked in groups. I have had some that have learned a lot by helping students in a group. I have learned to be cautious in grouping by different abilities. I tend to group students that I feel are at the same level in a course. Neither way is very good.