1. An apparatus (5) for generating an approximation function based on first pairs ((X.sub.1, Y.sub.1) to (X.sub.6, Y.sub.6)) of values associating a dependent variable (Y.sub.1 to Y.sub.6) with an independent variable (X.sub.1 to X.sub.6), and for determining second pairs (X.sub.A, Y'.sub.A) of values of said variables in accordance with said approximation function, characterized in that the apparatus comprises: first means (10): for iteratively determining at least one current linear regression function by making first errors of alternating sign equal in absolute value, which first errors have been measured between, respectively, first values (Y.sub.3, Y.sub.4, Y.sub.5) of the dependent variable for three pairs (X.sub.3, Y.sub.3) (X.sub.4, Y.sub.4) (X.sub.5, Y.sub.5) of a series of pairs, and second values (Y'.sub.3, Y'.sub.4, Y'.sub.5) of the dependent variable determined, in accordance with said current linear regression function, for the same values (X.sub.3, X.sub.4, X.sub.5) of the independent variable, for selecting that one of the current linear regression functions which produces an approximation of all the pairs of said series with a minimum error, and for coding the selected linear regression function with the aid of specific codes (p, q), and second means (17) for determining said second pairs (X.sub.A, Y'.sub.A) with the aid of said specific codes.