The X-bar chart tracks the mean of each subgroup, typically 4 or 5 consecutive parts, and responds to shifts between subgroups; the R chart tracks the range within each subgroup and watches short-term spread. Limits use tabulated constants: for subgroups of 5, A2 is 0.577 and D4 is 2.114. Read the R chart first, because the X-bar limits are built from the average range and mean nothing if the range is unstable.
R chart's clean but X-bar went seven above center after the slurry lot change. Pull the slurry CoA.
Above about 10 parts per subgroup, swap the R chart for an S chart: the range throws away too much information in large subgroups.
Also heard as
- Xbar-R
- X-bar R chart
- average and range chart
Related terms
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
Rational subgroup
Group of measurements chosen so that only routine, short-term variation can occur within it.
I-MR chart
Individuals and moving range chart: the control chart for one measurement per period, such as one batch or one lot.
Control limits
Boundaries on a control chart computed from the process's own variation, marking how far routine noise normally reaches.
Central limit theorem
Result that averages of many independent values tend toward a normal distribution even when the values themselves are not normal.