1. A method of monitoring a complex system having multiple sub-systems described by a plurality of operating parameters, comprising: continually generating data pertaining to said operating parameters during operation of said system; storing said data in a central database; defining a window of samples over which data is to be analyzed; normalizing said data to take into account variability factors and introduce a weighting factor to define thresholds dependent on the performance of individual monitored components and the performance of said components across the fleet; storing said defined thresholds for said defined window; and continually comparing new incoming data from said sub-systems with said stored defined thresholds to identify abnormalities in the system.