Concerns with the SDT approach to causal conditional reasoning: a comment on Trippas, Handley, Verde, Roser, McNair, and Evans (2014)

Signal Detection Theory (SDT; Wickens, 2002) is a prominent measurement model that characterizes observed classification responses in terms of discriminability and response bias. In recent years, SDT has been increasingly applied within the psychology of reasoning (Rotello and Heit, 2009; Dube et al., 2010; Heit and Rotello, 2010, 2014; Trippas et al., 2013). SDT assumes that different stimulus types (e.g., valid and invalid syllogisms) are associated with different (presumably Gaussian) evidence or argument-strength distributions. Responses (e.g., “Valid” and “Invalid”) are produced by comparing the argumentstrength of each syllogism with a set of established response criteria (Figure 1A). The response profile associated to each stimulus type can be represented as a Receiver Operating Charateristics (ROC) function by plotting performance pairs (i.e., hits and false-alarms) along different response criteria, which Gaussian SDT predicts to be curvilinear (Figure 1B). Trippas et al. (2014; henceforth THVRME) applied SDT to causalconditional reasoning and make two points: (1) that SDT provides an informative characterization of data from a reasoning experiment with two orthogonal factors such as believability and argument validity; (2) that an inspection of the shape of causal-conditional ROCs provides insights on the suitability of normative theories with the consequence to consider affirmation and denial problems separately. The goal of this comment is to make two counterarguments: First, to point out that the SDT model is often unable to provide an informative characterization of data in designs as discussed by THVRME as it fails to unambiguously separate argument strength and response bias. THVRME’s conclusion that “believability had no effect on accuracy [. . .] but seemed to affect response bias” (p. 4) solely hinge on arbitrary assumptions. Second, that THVRME’s reliance on ROC shape to justify a separation between affirmation and denial problems is unnecessary and misguided.