Control Chart - Difference between Average number and Average line?

 

Hi, The snapshot below shows the average 

 

 

average line differ from average number.jpg

The average number in the control chart above shown as 5d 8h but he average line red in the chart show more than 15d? They suppose to be identical?

1 answer

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Hi Thian,

When you have this chart setup to look at more than 30 days worth of data, this chart changes from using a linear scale for days into using a cube-root scale in regards to the vertical axis that measures days.

I would recommend checking out the documentation in regards to this in Viewing the Control Chart: Understanding the control chart.  In particular I would recommend expanding the section called 'Why does the scale of the Elapsed Time axis change when I change Timeframe?'

This example shows that when looking at this cube-root scale (which the chart does automatically for any time period above 30 days), the values are not as precise in regards to the location of the line.  I believe this is due to the scale itself and the magnitude of the data being displayed here.  In that example, the average is 4w 1d 10h average (which I believe translates to either 21 or 29 days).   So you can see that this average is between the vertical scale values of 20 and 40.   But I can understand why this is concerning, I think it might just be a limitation of the way this data has to be displayed.

Could you try something?  On that same chart, could you hover over a date such as Dec 3rd, where the rolling average (blue line) dips below the average (red line)?  I am curious to see what values actually appear there.   I believe that value will be below 14d, but I am interested to learn if it is also below the 5d 8h average there.   If it is, I think this would help confirm my thoughts that the location of that line/plot point is being skewed due to the cube-root scale is affecting this chart.

Please let me know,

Andy

Hi Andrew, thanks for the answer to confirm on my understanding.

When i hover over to Dec 3rd, the rolling average is 8.2days

Untitled.png

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