z-scores is a commonly used statistical method to evaluate when a measure is lower, within or higher the normal range.

Formula: z = (x-mean)/sd

AthleteMonitoring compares daily z-score to the 4-weeks rolling average, for each athlete and measurement, and alerts are calculated according to the range below:

Z-score< +-1= normal range- no alert
Z-score >0.99 <2= yellow alert
Z-score >1.99= red alert

Find more details about z-score here: http://en.wikipedia.org/wiki/Standard_score