An in-situ monitoring system is engaged with a mechanical system for which it is desired to assess or monitor the in-situ performance of the mechanical system. The in-situ monitoring system includes at least one input sensor and at least one output sensor for collecting data corresponding, respectively, to input into, and output from, the mechanical system. The monitoring system also includes an analyzer that utilizes the input and output data to generate an in-situ response representation of the mechanical system. The in-situ representation may be used to make a decision regarding the status of the mechanical system, e.g., to replace all or part of it, to adjust it or to do nothing. In one embodiment, the analyzer is programmed to adapt to the operating conditions of the mechanical system by automatically selecting which one of a number of target representations should be used to characterize the in-situ representation.