1. A method for evaluating a condition of an engine, the method comprising: obtaining a sample signature of a first engine, the sample signature characterizing the first engine as a function of a fluid sample of the first engine; comparing the sample signature to a plurality of reference signatures obtained from reference engines belonging to a common family with the first engine; selecting ones of the reference engines for which a difference between a corresponding reference signature and the sample signature is below a threshold; and providing historical engine data of the selected ones of the reference engines as a basis for evaluating the condition of the first engine; wherein the difference between the corresponding reference signature and the sample signature is found using: ∑ i = 1 n ⁢ W Ti ⁡ ( D Ei - D Ci ) 2 D Ei + D Ci where n is a number of data sets of the fluid sample, W Ti is a total weight assigned to a data set i, D Ei is a value of a data set i of the sample signature, and D Ci is a value of a data set i of the reference signature.