Metadata
- Author: Quality Digest
- Full Title:: Do You Have Leptokurtophobia?
- Category:: 🗞️Articles
- Document Tags:: A chart from the 40s is all you need
- URL:: https://www.qualitydigest.com/inside/quality-insider-column/do-you-have-leptokurtophobia.html
- Finished date:: 2024-02-22
Highlights
The symptoms of leptokurtophobia are (1) routinely asking if your data are normally distributed and (2) transforming your data to make them appear to be less leptokurtic and more “mound shaped.” If you have exhibited either of these symptoms then you need to read this article. (View Highlight)
One of these erroneous ideas was that you must have normally distributed data before you can put your data on a process behavior chart (also known as a control chart) (View Highlight)
When he created the process behavior chart, Shewhart (View Highlight)
(The first lesson of figure 1 is that three-sigma limits will filter out virtually all of the routine variation regardless of the shape of the histogram (View Highlight)
But that certainly makes the other limit look silly.” Yes, it does. Here we need to pause and think about those situations where we have skewed data. In most cases skewed data occur when the data pile up against a barrier or boundary condition. Whenever a boundary value falls within the computed limits, the boundary takes precedence over the computed limit, and we end up with a one-sided chart (View Highlight)
Thus, three-sigma limits work by brute force. They are sufficiently general to work with all types and shapes of histograms. They work with skewed data, and they work even when the limits are based on few data. (View Highlight)
New highlights added 2024-02-23
However, if we transform the data before we put them on a process behavior chart we end up with figure 9. There we find no points outside the limits! (View Highlight)
any high-order descriptive statistic that is computed globally is implicitly based upon a very strong assumption that the data are homogeneous. (View Highlight)
So how can we determine when a data set is homogeneous? That is the purpose of the process behavior chart! (View Highlight)