Mathematically calculated slope of standard curve, e.g., the plot of Ct values against logarithm of ten-fold dilutions of target nucleic acid. This slope is used for efficiency calculation. Ideally, the slope should be –3.32 (–3.1 to –3.6), which corresponds to 100% efficiency (precisely 1.0092) or two-fold (precisely, 2.0092) amplification at each cycle. Also called gradient.