Figure 2. An Illustration of the Ratcliff Diffusion Model.

Evidence starts accumulating at starting point z (inter-trial variation S_z). It accumulates with a drift rate mean of v and standard deviation of s (a scaling parameter not shown). The mean drift rate has an inter-trial variation of η. The evidence accumulates towards one of two decision thresholds, which are separated by the boundary separation a. An additional parameter is the non-decision time, T_er (inter-trial variation S_t).