Part Variation for Gage R&R Study

The estimate of actual part-to-part variation in Gage R&R, excluding the effect of measurement error, can be calculated using the formula here, or easily in your gage calibration and SPC software.

If # parts <10:

where:

Rpc-avg is the range between the average part measurements (i.e. using the average of each part, including all trials of the selected Appraisers, calculate the difference between the largest average and the smallest average part size).

Ordinate is the specified Sigma Level (usually 2.575 for Gage R&R analysis)

d2* is based on the sample size n = the number of parts and number of samples k = 1.

Notes:

1. Some authors, including AIAG, refer to the quantity (2*Ordinate/ d2*) as K3.

2. Complete tables for d2* values may be found in A.J. Duncan, Quality Control and Industrial Statistics, 1965, Third Edition Richard D. Irwin, Homewood, Illinois. Table D3, pg. 910.

If # parts ≥10:

where:

Spc-avg is the standard deviation of the average part measurements (i.e. using the average of each part, including all trials of the selected Appraisers, calculate the sample standard deviation of the average part sizes).

Ordinate is the specified Sigma Level (usually 2.575 for Gage R&R analysis)

c4* is based on a subgroup size equal to the number of parts and k=1 subgroups.

% PV is calculated using the Total Variation (TV):

% PV may also be calculated using the Part Tolerance:

where USL is the Upper Specification Limit and LSL is the Lower Specification Limit

Learn more about the Quality Improvement principles and tools for process excellence in Six Sigma Demystified (2011, McGraw-Hill) by Paul Keller, or his online Green Belt certification course ($499).