In order to provide an early and more accurate determination of network problems, current NPI OMs are compared with samples of recent historical NPI OMs so that changes in the NPI OM are detected based on current overall network conditions rather than on conditions that may have existed at statistically insignificant earlier operational periods. By constantly adjusting a performance threshold, against which the current NPI OM is compared, by using a smaller and very recent sampling of NPIs (in the case of sudden and abrupt performance-NPI degradation detection) or a larger and greater number of NPIs over a wider time priod (in the case of slow and persistent NPI degradation detection) to establish the threshold, detection results are more accurate and meaningful.