Specification limits are external tolerance values set by the user/customer and apply to individual units. Control limits are horizontal lines on a control chart that represents borderline for a process.
Control limits indicate the behavior of the process and whether the process is in control. The upper control limit (UCL) and lower control limit (LCL) are calculated statistically from the data available in the queries. UCL and LCL are set by calculating the mean, standard deviation, and range of process data collected for a stable process. Then, susequent data is compared to the calculated mean, SD, and range to determine whether the new data falls within the defined bounds. All the sample points fall within the control limits if the process is stable. Control limits define the area +/- 3 sigma on either side of the centerline. Data is normally distributed around a mean.
UCL is drawn above the centerline and called as +3 sigma line. LCL is drawn below the centerline and called as -3 sigma line.
The centerline is the mean value of the data set. The process is considered to be in control if data points fall within three standard deviations of the mean.
Graphical representation of control limits is shown below:
Specification limits are used to determine process capability and sigma value. You can define Upper Specification Limits (USL) and Lower Specification Limits (LSL) for each chart. Control limits are limits or averages that are calculated based on previous history.
Confusing process behavior
Calculated values of control limits are either narrow or flexible. Narrow values leads to process variation, and low quality whereas flexible values leads to misinterpretation of signals during process change, low quality, and higher costs.
Specification Limits | Control Limits |
---|---|
Voice of the Customer | Voice of the Process |
Values defined by user | Values calculated by Process |
Apply to subgroups | Apply to individual items |