Description:
Customers are often confused when creating rules using the conditions "Deviation from Normal" or "Time Over Dynamic Threshold" because of the confusing math involved to determine the correct conditional value for standard deviations. Instead of simply entering "above the mean by one standard deviation" or "outside the mean by 3 standard deviations," one must enter a percentile value. Further complicating things is that the valid value range for above/below conditions and outside conditions is different. This article confirms the proper values to enter.
Solution:
When dealing with standard deviations, what you are actually looking at is the percentage of polled data points that fall within a certain range on a bell curve. So for instance, if you set a rule condition to be "above the mean by one standard deviation," what this means is that 84.1% of the data points on the bell curve are below the upper limit of one standard deviation, and 15.9% of the data points are above, and Live Exceptions will alarm if the polled value falls in that 15.9% above range. Similarly, we can say the same applies to "below the mean" except that 84.1% of the values would be to the above the lower limit of one standard deviation and 15.9% would be below.
If you only want Live Exceptions to alarm if the polled value falls outside one standard deviation of the mean, that means it should alarm if the polled value falls into the 15.9% above one standard deviation, or the 15.9% below. Live Exceptions should not alarm if the polled value falls into the 68.2% in the middle of the bell curve.
Given this information, there are two sets of possible values to enter into a profile rule, depending on if you are creating a rule condition that is above or below the mean, or if the condition is outside the mean. These values are as follows:
For above the mean or below the mean:
1 standard deviation = 84.1
2 standard deviations = 97.5
3 standard deviations = 99.87
For outside the mean:
1 standard deviation = 68.2
2 standard deviations = 95.0
3 standard deviations = 99.74
A detailed spreadsheet listing all possible values is included with this knowledge document.