
322 A First Course in Quality Engineering
sensitive to deviation of the process distribution from normal as the X values do
not have the robustness present in the X values (central limit theorem)� Therefore,
the moving average and MR charts, to be discussed next, are preferred in these
circumstances� The X-chart, however, has some advantages; one of them is that
it is simple to use and easy to understand� Another is that specication lines can
be drawn on the chart and the characteristic can be monitored against the given
specications� Such specication lines, as we have mentioned before, should not
be drawn on charts for averages, because ...