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About this lesson
The run chart is the most common chart of process data. It is easy to create and maintain and gives the process operators immediate insight when a process becomes unstable.
When to use
If a run chart does not already exist, it should be immediately started during the Measure phase. The run chart should be maintained from that point forward until it is replaced by control charts in the Control phase.
A run chart is very easy to create and multiple run charts can be created for different process output parameters. The horizontal scale for the run chart is normally based upon a time increment such as hours or days, although it can also be sequential units. The vertical scale is based upon the parameter being tracked on the run chart.
The data points for the parameter are plotted. As soon as 15 points have been recorded, a median for the data set should be determined and shown as a horizontal line on the chart. The median can be recalculated periodically (monthly or after a significant number of additional units are included) but it should not change significantly unless there has been a process change or there is the presence of special cause variation.
The run chart indicates special cause variation if one of these five conditions exist:
- Nine consecutive points on the same side of the median
- Six consecutive points changing in the same direction (becoming larger or smaller)
- An astronomical point (a judgement call that the point is well beyond the normal expected values)
- Fourteen consecutive points of alternating higher then lower than the immediately preceding point in a sawtooth manner.
- Too many or too few “runs” based upon the number of “useful observations.” A useful observation is any point that is not the value of the median. A run is a set of one or more points on the same side of the median. The minimum and maximum number of runs that are consistent with only common cause variation must be looked at from a consecutive run table.
Hints & tips
- There is no reason to delay the start of a run chart – so don’t.
- If you have historical data, you can create a run chart from that data to see if there is special cause variation.
- Place the run chart at the point of data collection and plot it immediately so that the information is always up to date.
- Statistical process control charts should only be used with processes that are stable and under statistical control. That doesn’t apply to a run chart, so feel free to use it.
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