1. T20-02 Mean Chart (Unknown Variation) CL Calculations
Purpose Allows the analyst to calculate the "Mean Chart" for unknown variation 3-sigma control limits based on table lookup.
Inputs Sample Size Centerline (Grand Average) Average Range
Outputs Mean Control Chart LCL & UCL

2. Mean Chart ( Chart) - Unknown Variation
The Mean Chart Centerline, UCL and LCL (3 sigma limits) are calculated by the following when the variation of the process is unknown.

3. 3 Sigma Control Chart Table

4. Example
The mean of 20 samples each containing 5 observations are shown here. They have been taken from a process with unknown variance. The grand average can now be calculated. This can be done by using the Descriptive Statistics Template or more quickly by using the EXCEL built in AVERAGE function on this set of numbers.
Grand Average

5. Example
The range of the 20 samples are shown here. The average range can now be calculated. This can be done by using the Descriptive Statistics Template or more quickly by using the EXCEL built in AVERAGE function on this set of numbers.
Calculate the Control Limits for the Mean Control Chart.
Average Range

6. Input the Sample Size, Centerline (Grand Average), and Average Range in the light green cells.
The LCL, UCL are automatically calculated