Method of operating an apparatus for analyzing the condition of a machine part rotating with a speed of rotation, includes: receiving a first digital signal dependent on mechanical vibrations emanating from the rotating part; analyzing the first digital signal to detect peak amplitude values during a finite time period corresponding to a predetermined amount of revolution of the rotatable part; the predetermined amount of revolution corresponding to at least one revolution of the monitored rotatable part; defining a plurality of amplitude ranges, each amplitude range corresponding to a peak occurrence frequency of more than one peak per revolution; sorting the detected peak amplitude values into corresponding amplitude ranges to reflect occurrence of detected peak amplitude values within the plurality of amplitude ranges; establishing a peak amplitude value for detected peaks having an occurrence frequency of about NL peaks per revolution, the occurrence frequency NL being a number higher than one.