The equation has originally been proposed by Gladfelter for selective CVD . It has been derived for rate constants initially and Gladfelter showed that it can be used with coverages of GS and NGS. In the AS-ALD literature, however, several other approaches were developed to quantify the selectivity by quantities that are experimentally accessible. Mostly, "amount of material grown" is used instead of coverages (which are hard to determine experimentally). This amount is then determined as thickness, which is experimentally obtainable: (i) via the relative intensity of X-ray photoelectron spectroscopy (XPS) signals , energy-dispersive X-ray spectroscopy (EDS) mapping or as "substrate-normalized concentration" ; (ii) via ellipsometry . Ultimately, the selectivity is calculated as 𝑆 𝑥 = 𝑅 GS -𝑅 NGS 𝑅 GS + 𝑅 NGS .