A certain company has a complaint department that answers calls for three unique products. Because of failure rates inherent to each product type, some products have a higher expected complaint rate than others. Calls are tallied each hour and categorized by product type.

The department manager wants to use a single SPC control chart to track all of the calls. Because the complaint rate has already been established, the purpose of the control chart is to detect when the call rate has changed for any given product. What type of chart should be used?

A. A short run group c-chart, because the expected complaint rate is unique for each product type, but the tally window—one hour—is constant.
B. A target IX-MR chart, because this processing option will plot the data as a deviation from each product’s expected complaint rate.
C. A group np-chart, because the subgroup size is constant but the expected number of complaints is different within a given hour for each product type.
D. A group target Xbar-R chart, because the three product types can then be appropriately averaged but still remain as separate process streams using the group processing option.
E. All answers are statistically valid.
F. None of the answers are statistically valid.

The answer will be in the next issue of InsideSixSigma.