I would like to group data points of which I have several hundred thousand over a year's time. the data of interest is time per call. I have this information for every call. I want to look at it by week. If I use an Xbar R chart I would need to have the same sample size for every week. I won't because the call volume is variable. An I-MR chart would have too many data points. Is it valid to use an I-MR chart by putting down an average for each week as the individual data points?