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When
to Use an Attribute Chart
Only Attributes
data can be applied to an Attributes control chart.
To see the differences
between various attribute charts, let's consider an example of the errors
in an accounting process, where each month we process a certain number of
transactions.
- The Np-Chart monitors
the number of times a condition occurs, relative to a constant sample
size, when each sample can either have this condition, or not have this
condition. For our example, we would sample a set number of transactions
each month from all the transactions that occurred, and from this sample
count the number of transactions that had one or more errors. We would
then track on the control chart the number of transactions with errors
per month.
- The p-Chart monitors
the percent of samples having the condition, relative to either a fixed
or varying sample size, when each sample can either have this condition,
or not have this condition. For our example, we might choose to look
at all the transactions in the month (since that would vary from month
to month), or a set number of samples, whichever we prefer. From this
sample, we would count the number of transactions that had one or more
errors. We would then track on the control chart the percent of transactions
with errors per month.
- The c-Chart monitors
the number of times a condition occurs, relative to a constant sample
size. In this case, a given sample can have more than one instance of
the condition, in which case we count all the times it occurs in the
sample. For our example, we would sample a set number of transactions
each month from all the transactions that occurred, and from this sample
count the total number of errors in all the transactions. We would then
track on the control chart the number of errors in all the sampled transactions
per month.
- The u-Chart monitors
the percent of samples having the condition, relative to either a fixed
or varying sample size. In this case, a given sample can have more than
one instance of the condition, in which case we count all the times
it occurs in the sample. For our 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
count the total number of errors in all the transactions. We would then
track on the control chart the number of errors per transactions per
month.
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