Moving range charts display the difference between each successive pair of readings (e.g., difference between the first and second reading, the second and third, etc.). There will be one less moving range than there are individual readings (25 readings give 24 moving ranges).

Note: Even though the measurements are sampled individually, it is the number of readings grouped to form the moving range that determines the normal sample size n.

Data Available

n = number of readings grouped

D3, D4 = Subgroup size

k = number of samples

Central Line

n =2 is used for consulting the factor tables.

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Upper Control Limit

*

If

n = 2

D4 = 3.267

Lower Control Limit

*

If

n = 2

D3 = 0

There is no lower control limit for ranges for sample sizes below 7.

SPC chart calculations. |