1. A method comprising: detecting a feature of an identical input pattern using a plurality of feature detectors, each of which corresponds to a respective specific variation type and outputs a higher value when variation received by the input pattern roughly matches the respective specific variation type; determining a reference detector from among the plurality of feature detectors based on output values of the plurality of feature detectors; selecting a detector other than the reference detector from the plurality of feature detectors by comparing respective output values with an output value of the reference detector; and calculating a feature quantity of the input pattern based on an output value from the reference detector and the selected feature detector.