Page 126 - Six Sigma Demystified
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Chapter 5  m e a s u r e   s tag e        107



                             Project examPle: Process Baseline

                             A control chart of the order entry time (as defined earlier in the metric discussion)
                             is shown in Figure 5.6. The data were collected over a  six- week period. An initial
                             analysis using a  moving- average chart indicates that no groups are out of control.
                             A significant lack of fit to a normal distribution  (K- S value = 0.002) prompts the use
                             of a nonnormal Johnson distribution curve fit  (K- S value = 0.935). Seeking to explain
                             the wide  common- cause variation (technically part of the analyze phase), the data
                             were stratified by product. The team feels that this stratification suggests evidence
                             of variation between products, as shown by the differences between the diamonds,
                             filled circles, and triangles in Figure 5.6. Separate control charts for each product
                             revealed relatively in control yet skewed processes for each product. The causes for
                             the few  out- of- control subgroups (4 of the total 300) will be further considered in
                             the analyze stage; a discussion with the process personnel yielded no suggested
                             causes. The average order processing time across all products was calculated as 21
                             minutes, which corresponded well to the baseline costs noted in the charter. (Note
                             that although the median is the preferred statistic for describing central tendency
                             of a nonnormal distribution, the average provides a more direct indication of total
                             costs because the average cost times the number of orders equals the total cost. Use
                             of the median cost for estimating total cost is not so straightforward.)
                               A control chart for the errors is shown in Figure 5.7. In this case, a U chart was
                             used to plot the total errors each month. The team felt that this was the relevant




























                           Figure 5.6  Control chart baseline for example project’s order processing time stratified by
                           product.
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