“A common use of standard deviations is to periodically inspect the data set for a metric and alert if it has significantly varied from the mean. For instance, we may set an alert for when the number of unauthorized login attempts per day is three standard deviations greater than the mean. Provided that this data set has Gaussian distribution, we would expect that only 0.3% of the data points would trigger the alert. Even this simple type of statistical analysis is valuable because no one had to define a static threshold value, something which is infeasible if we are tracking thousands or hundreds of thousands of production metrics.†”
The DevOps Handbook
计算机存浮点数