z-score
How many standard deviations a value is above or below the mean.
The z-score of x is z = (x - μ)/σ for a mean μ and standard deviation σ; it puts readings from devices with different normals on one scale.
Examples
A detector that learned μ = 31.3 mg and σ = 2.1 mg scores a 44 mg window at z ≈ 6.0, above a raise line of 4.
The same 40 mg window scores z ≈ 4.1 on a stiff fan but z ≈ -3.8 on a fan whose normal is 55 mg, so one rule fits both.