Page 328 - Six Sigma Demystified

308 Six SigMa DemystifieD

Methodology

Samples are collected from the process at specific points each time. Each sam-
ple (at each point in time) has the same number of units, each of which must
either match or not match some criterion of interest (such as being defective).
Plotted statistic: The number of items in the sample meeting a criterion of
interest.
Centerline:
∑ m (count )
np = j =1 j
m
where m is the number of groups included in the analysis.
UCL, LCL (upper and lower control limits):

UCL = np + 3 np 1 − p)
(
np
LCL = max[0, np − 3 np 1 − p)]
(
np
where n is the sample size, np is the average count, and p is calculated as
follows:
∑ m (count )
p = j =1 j
×
m n
For example, we observe 500 telemarketing calls each week, and we record
the total number of calls with one or more errors. Ten months of data are
observed, with the following number of calls with errors in each sample: 7, 14,
12, 18, 9, 11, 12, 8, 11, and 21. The control chart is shown in Figure F.22.

Np Chart

Minitab

Use Stat\Control Chart\Attribute Charts\Np. Specify a constant Subgroup Size
for each group.

Options

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