Page 419 - Six Sigma Demystified

Part 3 S i x S i g m a To o l S 399

Figure F.55 Example of a U chart for varying sample size (created using Quality amer-
ica green Belt xL software).

where n is the sample size (number of units) of group j, and u is the average
j
percent.
For example, we observe 500 telemarketing calls each week and record the
total number of errors in each call, where each call may have more than one error.
Ten months of data are observed, with the following number of errors in each
sample: 7 of 350, 14 of 312, 12 of 125, 18 of 170, 9 of 165, 11 of 264, 12 of 254,
8 of 404, 11 of 137, and 21 of 312. The control chart is show in Figure F.55.

U Chart

Minitab

Use Stat\Control Chart\Attribute Charts\U. Specify a constant Subgroup Size or
a column containing variable Subgroup Sizes for each group

Options
Use Options\Display to specify plotted subgroups.
Use Scale\Time\Stamp to set column for x-axis labels.