Use/non-use reciprocity parameters. (a) A schematic and the equation used to calculate the root mean square of the successive differences (RMSSD). X(i) is the duration of a use or non-use epoch. There are (n−1) successive differences of use/non-use epochs. (b) A schematic and the equation used to calculate the Similarity Index, Xn(i) refers to the duration of a non-use (red) epoch, Yn(j) refers to the duration of a use (blue) epoch. Each non-use epoch (Xni) corresponds to three successive use epochs (Ynj, Ynj+1, Ynj+2). K is the number of non-use epochs in a day. Thus, the similarity index is the average absolute difference between the non-use and use epochs.