A method of operating an apparatus for analysing the condition of a machine having a part rotating with a speed of rotation, includes receiving a first digital signal dependent on mechanical vibrations emanating from rotation of the part, analyzing the first digital signal so as to detect peak amplitude values during a finite time period, the finite time period corresponding to a certain amount of revolution of the part, the certain amount of revolution corresponding to more than one revolution of the monitored rotatable part, defining amplitude ranges, sorting the detected peak amplitude values into corresponding amplitude ranges so as to reflect occurrence of detected peak amplitude values within the plurality of amplitude ranges, and estimating a representative peak amplitude value in dependence on the sorted peak amplitude values and the certain amount.