Capability Analysis (Poisson)
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Stat > Quality Control >Capability Analysis > Poisson

Use Capability Analysis (Poisson) to produce a process capability report when your data are from a Poisson distribution. Poisson data is usually associated with the number of defects observed in an item, where the item occupies a specified amount of time or specified space. The size of the item may vary, so you may also keep track of the size.

For example, if you manufacture electrical wiring, you may want to record the number of breaks in a piece of wire. If the lengths of the wire vary, you will have to record the size of each piece sampled. Or, if you manufacture appliances, you may want to record the number of scratches on the surface of the appliance. Since the sizes of the surface may be different, you may also record the size of each surface sampled, say in square inches.

Use Capability Analysis (Poisson) when your data meet the following conditions:

·    the rate of defects per unit of space or time is the same for each item

·    the number of defects observed in the items are independent of each other

Capability Analysis (Poisson) produces a process capability report for data from a Poisson distribution. The report includes the following:

·    U chart - verifies that the process was in a state of control at the time the report was generated

·    Chart of cumulative mean DPU (defects per unit) - verifies that you have collected data from enough samples to have a stable estimate of the mean

·    Histogram of DPU - displays the overall distribution of the defects per unit from the samples collected

·    Defect rate plot (when subgroup size is not constant) - verifies the assumption of the Poisson distribution that DPU is not influenced by the size of the items sampled

or

Poisson plot (when subgroup size is constant) - verifies that the Poisson distribution is appropriate for your data by plotting the expected number of defects against the number you actually observed

Dialog box items

Defects: Enter a column containing the number of defects from each item sampled.

Sample Size

Constant size: Choose to use a constant sample size for the analysis. Enter the value.

Use Sizes In: Choose to use a column containing the size of each item sampled. Enter the column.

Historical mu (Optional): Enter a historical value for the mean number of defects per unit. If you specify a value, Minitab uses it to assess capability. If you do not specify a value, Minitab uses the observed mean DPU of the sample to assess capablity.

Enter a target DPU for this process (Optional), Target: If you like, enter the target DPU.

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