The moving range measures how variation changes over time when data are collected as individual measurements instead of subgroups. It equals the range of two or more consecutive observations.
When data are collected as individual observations, you cannot calculate the standard deviation for each subgroup. In such cases, the average moving range and median moving range across all subgroups are alternative ways to estimate process variation. You can create a control chart of moving ranges to track process variation when you have individual observations.
|Response time||Range of values||Moving range, with length 2|
You may want to use moving ranges of different lengths if the data are cyclical. For example, if you collect quarterly data, you might use a moving range of length 4 to ensure that one observation from each season is included in the calculation. To do this, subtract the minimum value from the maximum value of four consecutive observations. If you want to calculate a moving range of length 4 for the above example, the first moving range value is 40 – 20 = 20.
The mean of the squared successive differences (MSSD) is used as an estimate of variance. It is calculated by taking the sum of the differences between consecutive observations squared, then taking the mean of that sum and dividing by two.
For cases when you can't assume that two successive points form a rational subgroup and use the moving range methods, the MSSD method provides an alternative. To use as an estimate of standard deviation, take the square root of MSSD.
|0.500 ml||0.480 ml||0.490 ml||0.500 ml|
|0.505 ml||0.500 ml||0.490 ml||0.498 ml|
|0.500 ml||0.479 ml||0.490 ml||0.510 ml|