1. A method comprising: determining characteristic parameters on the basis of a series of m values (H — 1, H — 2,. .. , H_m) over time, the values being descriptive for a predetermined process, the series having linear characteristics, the determining including: a) computing differences, denoted as Δ_i, between pairs of values (H_i, H_i+p) of points (t_i, t_i+p) of the historical time series, the points having a predetermined time distance to each other, denoted as p; b) determining the median value, denoted as Δ_i_M, of the computed differences Δ_i; c) computing, on the basis of the determined median value Δ_i_M, a trend parameter, denoted as T, T being defined as T=Δ_i_M/p.