Page 233 - The Six Sigma Project Planner

when it actually is acceptable. Continuing this evaluation shows that Inspector A made
the correct assessment 7 out of 8 times, for an accuracy of 0.875 or 87.5%. The results for
all inspectors are given in Table 21.
Table 21. Inspector Accuracies

Inspector A B C
Accuracy 87.5% 100.0% 62.5%

Repeatability and Pairwise Reproducibility
Repeatability is defined in Table 18 as the same inspector getting the same result when
evaluating the same item more than once within a short time interval. We see that when
InspA evaluated Part 1 in the morning of “Today,” she classified it as acceptable (1), but
in the afternoon she said it was unacceptable (0). The other three morning/afternoon
classifications matched each other. Thus, her repeatability is ¾ or 75%.
Pairwise reproducibility is the comparison of each inspector with every other inspector
when checking the same part at the same time on the same day. For example, for Part 1,
Morning, Today, InspA’s classification matched that of InspB. However, for Part 1,
Afternoon, Today, InspA’s classification was different from that of InspB. There are eight
such comparisons for each pair of inspectors. We see that InspA and InspB agreed
seven of the eight times, for a pairwise repeatability of 7/8 = 0.88.
In Table 22, the diagonal values are the repeatability scores and the off-diagonal
elements are the pairwise reproducibility scores. The results are shown for “Today,”
“Last Week,” and both combined.
Table 22. Repeatability and Pairwise Reproducibility for Both Days Combined

Overall Today Last Week

A B C
A B C A B C
A 0.75 0.88 0.50
A 0.50 0.75 0.50 A 1.001.000.50
B 1.00 0.50
B 1.00 0.75 B 1.000.50
C 0.25
C 0.50 C 0.00

Overall Repeatability, Reproducibility, Accuracy, and Bias
Information is always lost when summary statistics are used, but the data reduction
often makes the tradeoff worthwhile. The calculations for the overall statistics are
operationally defined as follows:

• Repeatability is the average of the repeatability scores for the two days combined,
i.e.,

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