The ratio of analytes is determined directly from the responses of the analytes using a conversion method. Individual analyte responses are obtained by using a selected measuring technique, and these individual responses are used as the independent variables in a conversion method. The dependent variable of conversion method is the desired analyte ratio. The resulting conversion method is then used to directly calculate the desired ratio of analytes as a function of the measured responses. No intermediate conversions, such as using a calibration curve to convert individual measured analyte responses to concentration values, are needed to obtain the desired ratio.