A machine monitoring device, the device comprising: (a) a communications link to interrogate a machine with a probe signal and receive one or more measured machine operating condition outputs reflecting the machine state characterized by machine operating condition input variables representing an individual parameter or set of parameters being considered; and (b) a device controller arranged for: (i) selecting a machine operating condition input variable (X n) for which a correlation with a corresponding machine operating condition output (Y n) is unknown; (ii) applying a predictive model in which the machine operating condition input variable serves as an argument of a predicted machine operating condition output, wherein the predictive model comprises at least two independent primary models, the primary models being used to create the predictive model, for each of which primary models at least one correspondence between a primary model machine operating condition (X 1, X 2) and a corresponding primary model machine output (Y 1, Y 2) are known; the primary models either initially share a common basis or are converted into primary models sharing a common basis before being incorporated into the predictive model if the primary models did not share a common basis prior to being converted into the predictive model; (iii) updating a library of predicted machine operating condition outputs (Xn, Yn); and (iv) alerting a human operator if one of the measured or the predicted machine operating condition output is at variance with a predetermined threshold machine operating condition output by comparing the machine operating condition output with a predetermined acceptable range of machine operating condition outputs. (a) a communications link to interrogate a machine with a probe signal and receive one or more measured machine operating condition outputs reflecting the machine state characterized by machine operating condition input variables representing an individual parameter or set of parameters being considered; and (b) a device controller arranged for: (i) selecting a machine operating condition input variable (X n) for which a correlation with a corresponding machine operating condition output (Y n) is unknown; (ii) applying a predictive model in which the machine operating condition input variable serves as an argument of a predicted machine operating condition output, wherein the predictive model comprises at least two independent primary models, the primary models being used to create the predictive model, for each of which primary models at least one correspondence between a primary model machine operating condition (X 1, X 2) and a corresponding primary model machine output (Y 1, Y 2) are known; the primary models either initially share a common basis or are converted into primary models sharing a common basis before being incorporated into the predictive model if the primary models did not share a common basis prior to being converted into the predictive model; (iii) updating a library of predicted machine operating condition outputs (Xn, Yn); and (iv) alerting a human operator if one of the measured or the predicted machine operating condition output is at variance with a predetermined threshold machine operating condition output by comparing the machine operating condition output with a predetermined acceptable range of machine operating condition outputs. (i) selecting a machine operating condition input variable (X n) for which a correlation with a corresponding machine operating condition output (Y n) is unknown; (ii) applying a predictive model in which the machine operating condition input variable serves as an argument of a predicted machine operating condition output, wherein the predictive model comprises at least two independent primary models, the primary models being used to create the predictive model, for each of which primary models at least one correspondence between a primary model machine operating condition (X 1, X 2) and a corresponding primary model machine output (Y 1, Y 2) are known; the primary models either initially share a common basis or are converted into primary models sharing a common basis before being incorporated into the predictive model if the primary models did not share a common basis prior to being converted into the predictive model; (iii) updating a library of predicted machine operating condition outputs (Xn, Yn); and (iv) alerting a human operator if one of the measured or the predicted machine operating condition output is at variance with a predetermined threshold machine operating condition output by comparing the machine operating condition output with a predetermined acceptable range of machine operating condition outputs.