2.4. Metrics of RNA Quantity
In [27] we define the relative hybridization degree, or S/N ratio, (3) Rp≡SpNp as the level of specific hybridization Sp of a probe set p relative to the baseline of non-specific binding Np of this probe set. Averaging these R values over all probe sets of a microarray exceeding a given expression threshold provides the relative specific transcript level, or mean log S/N ratio, (4) λ=〈log(R+1)〉R>0.5;chip⋅ 
We further defined β as the mean negative decadic logarithm of the non-specific signal of all probe sets of the array (5) β≈−〈logNp〉chip⋅ 
It was previously shown that β has a geometric interpretation as the width of the hook curve and describes the measuring range of specific hybridization [5].