1. A method for using input signal quality to improve speech recognition, comprising: obtaining quantitative measurements of the quality of an input signal of a speech recognition system, the quantitative measurements including at least a signal-to-noise ratio and a loudness of the input signal; analyzing the quantitative measurements to categorize the quality of the input signal into a qualitative category; and establishing a rejection threshold used in rejecting a speech recognition result for speech included in the input signal in dependence on the qualitative category of the input signal quality, the speech recognition result having a confidence score indicating a level of confidence in an accuracy of the speech recognition result, the rejection threshold establishing which confidence scores indicate that the speech recognition result is to be rejected as being incorrectly recognized.