1. A method of generating delay values for instances of a circuit, comprising: on a programmed processor, performing operations including: inputting a plurality of contexts, wherein each context includes a respective delay value and a combination of possible types of a plurality of characteristics of the circuit, and each characteristic is of one type of a plurality of alternative types of the characteristic; inputting a plurality of classification parameters, the classification parameters indicating selected ones of the characteristics, and each of the plurality of characteristics not indicated by a classification parameter being a non-selected characteristic; selecting groups of contexts based on the plurality of classification parameters, wherein: each group includes one or more of the contexts, and each context includes the plurality of characteristics; and a combination of types of the selected characteristics in each context in a group is equal to the combination of types of the selected characteristics of each other context in the group; determining for each group and from the contexts in each group, a mean and a standard deviation of the respective delay values; and outputting the mean and standard deviation of each group.