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|Table of Contents Statistical Tests Control Charts x- and R-Charts|
|See also: control charts, p-and c-charts|
The x-chart shows the average of n samples drawn from a process in constant time intervals. The figure below shows the x-chart of a steel plate production line. 4 steel plates are drawn from the manufacturing process every 6 hours. The thickness of the four samples is measured and the average is entered into the control plot. The control limits are set in such a way that the chances of the mean falling outside them are well below 1 %. In fact, the control limits are set such that they span a range between and .
Another control chart which is quite similar to the x-chart, but which
shows the variability of the process, is the R-chart. The R-chart
plots the range of successive samples along the time axis. The lower and
upper control limits of an R-chart are again determined in such a way that
the probability that a particular R will fall outside the control limits
is low (usually below 1%).
Last Update: 2006-Jšn-17