Several previous discounting studies have used the R 2 measure to identify data sets with poor fits to a mathematical discounting model as nonsystematic data to be eliminated before further analyses are conducted. Data from three previous delay-discounting studies (6 separate groups, with a total of 161 individuals) were used to demonstrate why using R 2 to assess the fits of discounting data is problematic. A significant, positive correlation between discounting rate parameter and R 2 was found in most groups, showing that R 2 is more stringent as a measure of fit for low discounting rates than for high discounting rates. Furthermore, it is suggested that identifying nonsystematic data based on any measure of fit to a mathematical discounting model may be problematic because it confounds discounting rate comparison with the issue of discounting model assessment. Therefore, a modelfree method to identify nonsystematic data is needed. An algorithm for identifying nonsystematic data is presented that is based on the expectation of a monotonically decreasing discounting function. This algorithm identified 13 cases out of the 161 reanalyzed data sets as nonsystematic. These nonsystematic data are presented, along with examples of data not identified as nonsystematic. This algorithm, or modifications of it, may be useful in a variety of human and nonhuman animal discounting studies (e.g., delay discounting, probability discounting) as an alternative to the R 2 measure for identifying nonsystematic data. The algorithm may be used in empirical investigations to improve discounting methodology, and may be used to identify outliers to be removed from analyses.Keywords delay discounting; probability discounting; hyperbolic discounting; R 2 ; outliers Delay discounting (also referred to as "time" or "temporal" discounting) is the concept that delaying a consequence decreases its effect on behavior (Critchfiled & Kollins, 2001). Such devaluation of delayed consequences has been demonstrated in both human and nonhuman animals, typically with choice procedures revealing that, all else being equal, sooner reinforcers are preferred over delayed ones, and delayed losses or punishments are preferred over sooner ones (e.g., Ainslie, 1975;Baker, Johnson, & Bickel, 2003;Benzion, Rapoport, & Yagil, 1989; Chung & Herrnstein, 1967;Deluty, 1978;Green, Fisher, Perlow, & Sherman, 1981;Green, Fry & Myerson, 1994;Loewenstein, 1988;Logue, 1988;Mischel, Shoda & Rodriguez, 1989;Rachlin, Raineri, & Cross, 1991;Rachlin & Green, 1972; Shelly, 1993;Thaler, 1981 Delay discounting is considered a behavioral model of impulsivity, and the use of laboratory choice procedures may represent the best way to empirically quantify delay discounting (Ainslie, 1975).Application of delay-discounting methods and analyses has rapidly expanded to address a variety of issues of clinical and theoretical importance. For example, several drug-dependent populations discount future consequences at high rates, indicating that delay discounting may serve a fundame...