Cartoon illustrating how the noise parameter is defined. a) Three notional noisy spectra are shown together with the notional grand average of all the entire hypothetical dataset, assumed to be noiseless. Example 1 (circles) is basically superimposed on the grand average; example 2 (triangles) is displaced, but with roughly the same trend of the grand average; and example 3 (squares) has a trend different from the grand average. b) The difference between each of the three spectra, shown in panel a) and the grand average; the three straight lines represent the linear fit of each of the three differential spectra. On the right side are shown the three differences between the linear fits and the data shown in panel b), relative to example 1 (panel c)), example 2 (panel d)), and example 3 (panel e)), respectively.