Is there a statistical reason why item analysis/response theory isn't more widely applied? For instance, if a teacher gives a 25 question multiple choice test and finds that 10 questions were answered correctly by everyone, 10 questions were answered by a really low fraction (say 10%) and the remaining 5 were answered by roughly 50% of people. Doesn't it make sense to reweight the scores so that hard questions are given more weight?
And yet, in the real world tests almost always have all questions weighted equally. Why?
The below link discusses discrimination indices and other measures of difficulties for choosing which questions are best: http://fcit.usf.edu/assessment/selected/responsec.html
It seems though that the method of figuring out the discrimination index of questions is only used in a forward-looking way (eg., if a question doesn't discriminate well, toss it). Why aren't tests re-weighted for the current population?