Individuals and moving range chart: the top panel plots each single reading and the bottom plots the absolute difference between consecutive readings. Sigma is estimated as the average moving range divided by 1.128, so the individuals limits sit at the mean plus or minus 2.66 times the average moving range. It suits batch chemistry, daily monitors and one-wafer-per-lot data, but it reacts badly to skewed data and autocorrelation.
Only one monitor wafer a day on that implanter, so it's an I-MR chart, not X-bar and R.
Estimate individuals sigma from the average moving range, never from the overall standard deviation: a shift in the data inflates the overall figure and hides itself.
Also heard as
- individuals chart
- XmR chart
- I chart
- ImR
Related terms
X-bar and R chart
Paired control charts for subgroup averages and subgroup ranges, the workhorse for measured data sampled in small groups.
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
Autocorrelation
Correlation between successive values in a time series, which breaks the independence that standard control charts assume.
EWMA
Exponentially weighted moving average: a chart or controller that blends each reading with the past to catch small, slow shifts.