a direction of change 56 57 and if that is the case increases the counter 32 b which is associated with the threshold value in question step 49 the program control then reverts to step 44 by having two change threshold values t 1 t 2 each counter 32 b thus represents an open interval 58 59 in a direction of change 56 57 counted from the threshold values t 1 t 2 according to one embodiment one change threshold value t 1 is greater than zero and represents increases and one change threshold value t 2 is less than zero and represents decreases these threshold values then define an interval 53 and changes outside this interval are recorded as increases of counter values the interval 53 can represent changes which are considered as normal for the signal according to another embodiment of the invention only one threshold value is defined and compared with the absolute value of the measured change according to a further embodiment of the invention at least two threshold values t 1 t 3 are defined in at least one direction of change 56 57 which threshold values define an interval 54 in this case a counter is associated with the interval 54 and a counter is associated with the open interval 55 the number of threshold values as well as the number of value intervals 51 can of course vary for different signals more values yielding a better degree of resolution of the information about changes of a signal fig6 is a flow diagram showing how the step 47 of measuring a change of a signal can be realised in step 60 it is determined whether the sampling in question is the first.