We have clients come to us and say here's our average customer: female, she's 34 years-old, she has 2.3 kids....we listen politely and nod and say well that's great but we don't care about that person. What we really need to do to design is look at the extremes: the weakest or the person with arthritis or the athlete or the strongest or the fastest person. Because if we understand what the extremes are, the middle will take care of itself.Again with feelings..if we understand what the extremes are, the middle will take care of itself.
An industrial designer has summed up very nicely my feelings on both educational research and teaching in general.
So much in educational research goes to the average kid. We look at the effect size. We do meta-analyses. We care about how a giant lump of kids does on average against another giant group of kids. What we should care about are the extremes. A .4 effect size is nice. What I really care about is the outliers in the group. What type of student did this treatment make a HUGE effect on? It gets washed out because of our love of averages. When we look at research, we shouldn't care so much about the stuff that makes the group move a little on average (although that's still good to know). What we should be looking at is those students or groups of students in the treatment that were extremely positive and negative. What conditions existed for this specific student to make such an extreme gain? We can move towards truly individualized education only when we can say what works for this specific student under these specific conditions.
As for teaching, I don't know how many times I've been told by a principal/other teacher/staff developer that I should choose 3 or 4 kids in the middle and design my instruction around them. I've done that for a long time and had pretty good results. I'm becoming convinced though that I need to be more like Dan and design for the extremes. I need to take care of the really high and the really low and the middle will take care of itself.