Like many of you, I took statistics in school. And, if you’re like me, stDev sounded interesting, but you were not yet able to turn it into a practical use. Here’s how :
First, there’s two stdDev functions : stdDev and stdDevP.
If the list of data you’re performing this function on is complete, then you use stdDevP, otherwise, when you only selected a part of the data, use stdDev.
The way it is calculated is this :
First, every number is subtracted from the average (mean) and squared.
5,3,7 average = 5
5 – 5 = (0)² = 0
3 – 5 = (–2)²= 4
7 – 5 = (2)² = 4
Next, an average is taken and square-rooted.
sqrt(8/3) = 1,632 and that’s how much, on average, those three numbers vary from eachother.
So, if a number is higher than the average + stddev, it is statistically significatly higher than the average.
If the number is lower than the average – stddev, it is statistically significantly lower than the average.
So, now you can create charts, alerters etc using those two numbers :
Above Avg+stddev = Green
Between Avg+stddev and Avg-stddev = white
Below Avg-stddev = Orange