Thank you (all 6 of you) for your answers. Through trying 2 of them, I've realised that I have asked the wrong question, or at least, what I assumed, in asking the question, was wrong.
Each group for which I have all the data consists of a normal distribution. I was assuming that, if I were to combine the data from all the groups together, then I could get another normal distribution. I required the psychometric function of this combined data, which I intended to calculate from the combined mean and SD, assuming normal distribution. However, even if all the individual groups exhibit a normal distribution (and I don't have data to prove this!), the combination certainly doesn't have to - and may not even be close if I choose to combine two groups with very different means (I believe this is one issue hinted at in the final post). I have chosen to calculate the psychometric functions on a group by group basis. Thus, I have the %age of people in each group who would respond to a sound of a given intensity. Then, it is trivial to add the percentages across groups (at each intensity) weighting by the number of subjects in each group, or some other factor. I hope this isn't a terrible thing to do? I want to combine the data from different groups because: a) I have data from experiments that should be the same, but aren't! b) I have data from different age ranges, which should be different (and are!), but I wish to calculate %age values for the whole population (with a given demographic, which I can create using correct weightings of each age-band data-set). I sincerely apologise for asking the wrong question. If my chosen method is totally indefensible, I hope you will correct me. Thank you for your help. Cheers, David. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
