The SPC Procedure
Constructing Charts for Means
You produce control charts for means by using the BOXCHART, XCHART, XRCHART, or XSCHART statement. The following notation is used in this section:
Process mean (expected value of the population of measurements) | |
Process standard deviation (standard deviation of the population of measurements) | |
Mean of measurements in ith subgroup | |
Sample size of ith subgroup | |
Range of measurements in ith subgroup | |
Sample size of ith subgroup | |
N | Number of subgroups |
Weighted average of subgroup means |
Subgroup Summary Statistic
Each point on an chart indicates the value of a subgroup mean (). For example, if the 10th subgroup contains the values 12, 15, 19, 16, and 14, the subgroup summary statistic is
Central Line
By default, the central line on an chart indicates an estimate for , which is computed as
If you specify a known value () for , the central line indicates the value of .
Control Limits
The limits are computed as a specified multiple (k) of the standard error of above and below the central line. The default limits are computed using k = 3 (these are referred to as limits).
Table 20.19 provides the formulas for the limits.
Note that the limits vary with . If the standard values and are available for and , respectively, replace with and with in Table 20.19.
You can specify parameters for the limits as follows:
Specify k by using the SIGMAS= option or the variable
_SIGMAS_in a LIMITS= data table.Specify a constant nominal sample size for the control limits by using the LIMITN= option or the variable
_LIMITN_in a LIMITS= data table.Specify by using the variable
_MEAN_in a LIMITS= data table.Specify by using the variable
_STDDEV_in a LIMITS= data table.