1. A method of assessing the performance of a mechanical system having an input loading and a response to the input loading, comprising: storing, in a memory associated with a performance analyzer via a first signal from a first sensor placed in operative relation to the mechanical system, input data corresponding to the input loading, wherein the first sensor is not an operational part of the mechanical system; storing, in the memory associated with the performance analyzer via a second signal from a second sensor placed in operative relation to the mechanical system, response data corresponding to the response, wherein the second sensor is not an operational part of the mechanical system; automatically generating in the performance analyzer an in-situ response representation as a function of the input data and the response data; automatically selecting in the performance analyzer one of a plurality of stored target-response representations as a function of the input data; automatically making in the performance analyzer a comparison of the in-situ response representation to the one of the plurality of stored target-response representations for a purpose other than controlling the response of the mechanical system; and outputting to a user from the performance analyzer information concerning said comparison.