Pair comparison methods based on Case V of Thurstone's Law of Comparative Judgment are widely used to derive interval scales for perceptual image quality. A thorough treatment of the involved statistical errors is often neglected, even though this is the base for computing confidence intervals and other statistical tests. In this paper we show, that consequent error estimation through all steps of the data analysis provides a simple and reliable method to compute confidence intervals. Monte Carlo simulations are used to verify the results and to compare the proposed error estimation with other known methods.