Testing the convergent validity of the nondecision time parameter of the diffusion model

Modeling reaction time data using diffusion models comes with the advantage of separating different processes involved in decision making. Each parameter of the diffusion model is assumed to translate into one (or more) process(es). If this assumption holds true, selective manipulations of specific processes involved in decision making should selectively influence the related parameter, but not any other parameter (high convergent and discriminant validities). We present two experiments (total N = 104) and one simulation study that-using a manipulation of the difficulty of encoding in a lexical decision task-tested the convergent validity of the nondecision time t₀. As hypothesized, we found large effects on t₀ in both experiments but also medium to large effects on the drift rate v as well as on the starting point z. Our simulation study suggests that the effects on the drift rate, but not on the starting point, might be explained by incorrect assumptions about the intertrial variability of the nondecision time. Our results speak in favor of a high convergent validity of t₀ but question the discriminant validities of v and z, at least under the assumption that our manipulation affected encoding selectively. (PsycInfo Database Record (c) 2026 APA, all rights reserved).