It is not easily attainable, in both cost and time aspects, to regularly monitor the height keeping performance of all aircraft operated by every operator. ICAO Asia/Pacific region has determined the Minimum Monitoring Requirements (MMR) to sample, for each operator, an adequate number of aircraft per type and series. The MMR are given in tables indicating types and series of aircraft, the number of sample aircraft from each fleet and the monitoring period. See MMR document for details.