The fact that the subgroup size is large does not invalidate the various control charts. In fact, everything else being equal, more data is almost always better than less data!

With large subgroups, S charts give a much better estimate of the standard deviation than R charts would, so I agree with Benjamin's suggestion for Xbar-S charts.

With such a large subgroup, you *will *get very narrow control limits on the X-bar chart. However, when you are averaging so many points, the average *should *be quite close to the true mean. If the Xbar chart shows many OOC points, that means that many of the subgroups do indeed show more variation than would be expected due simply to random variations. With big subgroups, you can detect small variations. Now, these small variations that are statistically significant may not be economically significant.

If you expect some variations between subgroups in addition to the variations within a subgroup, then you might calculate the average for each subgroup and plot that set of points on an I-MR chart. Basically you are admitting that there is long-term variation that is not accounted for within the subgroups, and you are just looking at the long-term trends. An OOC point on the I chart tells you that a subgroup's mean shows an unusual amount of variation (and the usual variation is already larger than predicted within the subgroups).

Tim F