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.
= 
=
| 
- 
|
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. |
