Page 418 - Six Sigma Demystified
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398 Six SigMa DemystifieD
than one instance of the condition. For example, we might choose to look at all
the transactions in the month (since that would vary month to month) or a set
number of samples, whichever we prefer. From this sample, we track the total
number of errors in each month.
When to Use
Measure Stage
• To estimate, using attributes data, the process baseline (Generally, we
would greatly prefer the use of variable control charts for this purpose.)
Improve Stage
• Since the number of errors tends to be quite small (for even very large
samples), the use of attributes charts is limited in the improve stage
Methodology
Samples are collected from the process at specific points each time. Each sam-
ple (at each point in time) may have a unique number of units, each of which
may have one or more errors.
Plotted statistic: The average count of occurrences per unit of a criterion of
interest in a sample of items.
(count )
u = j
j n j
where n is the sample size (number of units) of group j.
j
Centerline:
∑ m (count )
u = j =1 j
m
where n is the sample size (number of units) of group j, and m is the number
j
of groups included in the analysis.
UCL, LCL (upper and lower control limits):
u
UCL = u + 3
n j
u
LCL = max 0, u − 3
n j 
