On the analysis of psychometric functions: The Spearman-Karber method

With computer simulations, we examined the performance of the Spearman-Karber method for analyzing psychometric functions and compared this method with the standard technique of probit analysis. The Spearman-Karber method was found to be superior in most cases. It generally yielded less biased and less variable estimates of the location and dispersion of a psychometric function, and it provided more power to detect differences in these parameters across experimental conditions. Moreover, the Spearman-Karber method provided information about the skewness and higher moments of psychometric functions that is beyond the scope of probit analysis. These advantages of the Spearman-Karber method suggest that it should often be used in preference to probit analysis for the analysis of observed psychometric functions.