Page 222 - Six Sigma Demystified

202 Six SigMa DemystifieD

C Chart

The C chart is one of a set of control charts specifically designed for attributes
data. It monitors the number of times a condition occurs relative to a constant
sample size when each sample can have more than one instance of the condi-
tion. For example, a specified number of transactions each month can be sam-
pled from all the transactions that occurred. From each sample, you can track
the total number of errors in the sample on the C chart.

When to Use

Measure Stage
• To estimate, using attributes data, the process baseline (Generally, we
would greatly prefer the use of variables control charts for this purpose.)

Improve Stage

• Since the number of errors tends to be quite small (even for very large
samples), the use of attribute 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) has the same number of units, each of which may
have one or more errors.
Plotted statistic: The count of occurrences of a criterion of interest in a sample
of items
Centerline:
∑ m (count)
c = j =1 j
m

where m is the number of groups included in the analysis. UCL and LCL are
the upper and lower control limits:

UCL = c + 3 c
LCL = max 0, c − 3 c )
(

where n is the sample size and ˉc is the average count.