The proposed prediction of trends is based on history values of an observable. In more detail, a mathematical model is fitted on history values of an observable, said mathematical model having a ratio factor defined as a ratio between a number of history values used for calculation of a given observable value and a number of observable values modelled simultaneously. Information defining a minimum forecast horizon and a maximum forecast horizon together with respective ratio factor values is received, together with information defining a desired forecast horizon. A desired ratio factor value corresponding to the desired forecast horizon is determined based at least on said desired forecast horizon and said minimum and maximum forecast horizons together with said respective ratio factor values. The desired ratio factor value is used in fitting said mathematical model on said history values of said observable.