Avinash posted today on “Leveraging Statistical Control Limits” in web analytics as a way to cut out the noise in your KPIs and therefore being able to focus on the true outliers – data points that are truly worthy and in need of your valuable brain power.
In his post, Avinash challenged his readers to provide an Excel solution since there are no OOTB (Out Of The Box) solutions for introducing constants, statistical or otherwise, into charts – Excel tries to box us into using just vertical and horizontal axes scale controls.
Well, I can’t pass up a challenge like that right?
What do you think of this?
So what’s going on here?
- Named Ranges – Named ranges are your friend. In this case, I’ve got named ranges for each important series of data, especially my key metric – page views per visit. That way, I can insert a new row of data and my constants (Standard Deviation, Mean, Upper Control Limit, and Lower Control Limit) will update automatically. BTW, the chart uses the named ranges as well so that it will update automatically every time a new row of data is added.
- The chart is a combination of lines and stacked bars. The stacked bars create the gray biorhythm (normal variation) area. Line charts are used for everything else.
- For the top 5 outliers (either upper or lower) I’m dynamically adding a data label so that I know, at a glance, the specific values for them.
- I’ve eliminated all horizontal lines other than my own constants because they are just distracting
You can download the spreadsheet and play with it if you want.
A couple of caveats:
- Named ranges won’t do the dishes for you. In other words, when you insert a new row, you’ll need to copy down the formulae from the previous row
- Excel doesn’t always do a good job of placing data labels so the outlier labels may need to be massaged after an update
- My spreadsheet only uses 1 standard deviation. While 3 might be a six-sigma best practice, three standard deviations would have put the lower control at 0.1 and common sense tells me that it shouldn’t be possible to have an average page views per visit value lower than 1.
I’ve deliberately not gone into great detail on the inner-mechanics of the spreadsheet (not that their profound or anything) because that would make for an impossibly long post – if you have a question please post it in the comments or email me directly (civy at instantcognition.com).