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Time Weighted Control Charts
There are several types of time weighted SPC charts that modify the current data value with historic data. These are used with variable data parameters to detect small shifts in the mean.
When to use
These charts are often used after a process has first been brought under statistical control by using a different chart. These are then employed to detect small shifts in the mean, especially if the process has high levels of common cause variation creating noise in the system.
Time weighted control charts are created using the control chart process discussed in an earlier module. The data on these charts is modified from the actual subgroup data to include as aspect of historical data. These control charts were recent additions to the family of statistical control plots and they do not follow the same pattern as the earlier Shewhart control charts. The X axis is still sequential time or subgroups. The Y axis is the modified time-weighted value. These charts are normally centered vertically on the mean value (or target value). They still include upper and lower control limits.
While there are many time weighted control charts, the two most commonly found in business are the CUSUM and the EWMA. Both will detect small shifts to the mean better than the traditional Shewhart control charts.
- CUSUM is better at detecting long term drift in the mean.
- EWMA is better at detecting near term drift in the mean.
Because of the weighting, these charts are less susceptible to noise in the system. That is why they are an excellent choice for monitoring the mean with a system that has high common cause variation. These charts will tend to suppress the impact of that noise.
The calculation needed to create the data set and the control limits for these charts is more complex than with the Shewhart control charts. While it is difficult to do these manually, most statistical software applications have the formulas and can do the calculation and create the control chart plots.
Hints & tips
- These are a good follow-on control chart to use once a process is under statistical control. These will quickly identify small shifts in the mean. The down side is that they will be slightly slower to detect large shifts.
- The math on these can get tricky. If you are planning to use one of these charts, I strongly recommend Minitab or another statistical software application.
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