“…In the case of the DJIA, two data pre-processing schemes (also called normalizing, or data transformation), and , are considered: (i) subtracting the arithmetic average and dividing by the standard variation, that is by calculating , where and , and (ii) by applying a logarithm so that . The linear transformation is often adopted in statistics and signal processing [ 64 , 65 , 66 , 67 , 68 ], while the non-linear transformation can be adopted with signals revealing an exponential-like evolution [ 69 , 70 , 71 , 72 , 73 ]. Of course, other data transformations could be envisaged, but these two are commonly adopted.…”