Does data have to be normal for control charts?

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.

What is non normal data?

Non-normality is a way of life, since no characteristic (height, weight, etc.) will have exactly a normal distribution. One strategy to make non-normal data resemble normal data is by using a transformation. These transformations are defined only for positive data values.

What to do when data is non normal?

Many practitioners suggest that if your data are not normal, you should do a nonparametric version of the test, which does not assume normality. From my experience, I would say that if you have non-normal data, you may look at the nonparametric version of the test you are interested in running.

What is the difference between a normal and a non normal control chart?

No one understands what the control chart with the transformed data is telling them except whether it is in or out of control. Non-normal control chart: This involves finding the distribution, making sure it makes sense for your process, estimating the parameters of the distribution and determining the control limits.

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.

Do you need to normalize data to draw a control chart?

You don’t need to normalize data to draw a control chart or a histogram. You just need to know what you’re trying to accomplish through improvement: move the center and reduce the variation. Are there situations where you will need to normalize data? Sure, but in 25 years of process improvement I haven’t needed to do it yet.

When do you need an alternate control chart?

Non-normal Data Needs Alternate Control Chart Approach. Some practitioners mistakenly believe that it is not necessary to transform data before creating an individuals control chart when the underlying process distribution response is not normal. An individuals control chart, however, is not robust to non-normally distributed data.