How is the rolling average on the Control Chart calculated?

[Katerina Blakhava]

How is the rolling average on the Control Chart calculated?

Is there any specific approach to choosing a set number of issues? Or is it always 20% of all the issues? 

[Sam Hall]

Hi Katerina,

There's some information here: https://confluence.atlassian.com/jirasoftwarecloud/control-chart-777002660.html

From the section "How is rolling average calculated?":

The rolling average (blue line on the chart) is issue-based, not time-based. For every issue shown on the chart, the rolling average (at that point in time) is calculated by taking the issue itself, X issues before the issue and X issues after the issue, then averaging their cycle times. 20% of the total issues displayed (always an odd number and a minimum of 5 issues) is used in the calculation.

For example, in the screenshot below, at the point of time where an issue (green dot) is shown, the rolling average is calculated as follows:

1. Take the issue plus four issues before and four issues after (nine issues total).
2. Average the cycle times for the nine issues.
3. Map the blue line to the calculated average.

If the Timeframe is reduced to 'Past two weeks', the number of issues used would reduce, as there are fewer total issues available to use for the calculations.

rolling-ave.jpg

This method produces a steady rolling average line that shows outliers better (i.e. rolling average doesn't deviate as sharply towards outliers). The rolling average line is also easy to understand, as the inflections are related to the positions of issues.

 If you would like to know more about why the rolling average calculation is based on a percentage of the total issues, rather than a time period, see Comparing different methods of calculating the rolling average on the Control Chart.

Hope this helps.

Sam

[Katerina Blakhava]

hi Sam,

Thank you for the answer!

Anyway my question is, how do we estimate X issues before and after the one we are working on? I've studied the article but don't find this information

[Sam Hall]

I think you can work it out from the rolling average issue window.

In the example, it says "9 issue window" - so that means the rolling average is calculated using 4 issues before and 4 issues after the one in question.

If it says "5 issue window", which is the minimum it can be, the rolling average is calculated using 2 issues before and 2 issues after the one in question.

Sam

[Paul Johnson]

A question in relation to the rolling average calculation ; if I am at the end of the reporting period (today) , there will be no issues after the current issue. How is the rolling average then calculated?