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Do you need normal distribution for control chart?
Data do not have to be normally distributed before a control chart can be used – including the individuals control chart. But, you better not ignore the distribution in deciding how to interpret the control chart. This month’s publication examines how to handle non-normal data on a control chart –…
How to conduct capability analysis with nonnormal distribution?
In Minitab, select Stat > Quality Tools > Capability Analysis > Nonnormal… Populate the “Capability Analysis (Nonnormal Distribution)” dialogue box as seen below. Make sure to select “Exponential” next to Fit distribution. Then, Click on “Options”. Fill in the “Capability Analysis (Non Normal Distribution): Options” dialogue box with the following:
Can you use statistical control on non-normal data?
Note that this chart is in statistical control. In addition, there are no false signals based on runs below the average (note: with a larger data set, there probably would be some false signals). Subgrouping the data did remove the out of control points seen on the X control chart. So, this is an option to use with non-normal data.
How to deal with non-normally distributed data?
Another approach to handling non-normally distributed data is to transform the data into a normal distribution. For example, you can use the Box-Cox transformation to attempt to transform the data. The data were transformed using the Box-Cox transformation. The rounded value of lambda for the exponential data is 0.25.
When do control charts of non-normal data work?
“All interpretation is faulty when the data distribution on an individual X chart is not normal” and also the Central Limit Theorem: “The distribution of sample means approaches normal regardless of the shape of the parent population. This is why AverageX-R control charts work!” needs work.
What happens when data is not from a normal distribution?
If data is not from a normal distribution, an individuals control chart can generate many false signals, leading to unnecessary tampering with the process.
How is the assumption of normality used in a control chart?
The assumption of normally distributed data is introduced into the control chart formulas through the use of the control chart constants. The values for these constants are computed using a normal probability distribution for the original data. Fortunately, the assumption of normality is not a critical assumption.