The effects of aging on the speed–accuracy compromise: Boundary optimality in the diffusion model.

Abstract

We evaluated age-related differences in the optimality of decision boundary settings in a diffusion model analysis. In the model, the width of the decision boundary represents the amount of evidence that must accumulate in favor of a response alternative before a decision is made. Wide boundaries lead to slow but accurate responding, and narrow boundaries lead to fast but inaccurate responding. There is a single value of boundary separation that produces the most correct answers in a given period of time, and we will refer to this value as the reward rate optimal boundary (RROB). Across a variety of decision tasks, we consistently found that older adults used boundaries that were much wider than the RROB value. Young adults used boundaries that were closer to the RROB value, although age differences in optimality were smaller with instructions emphasizing speed than with instructions emphasizing accuracy. Young adults adjusted their boundary settings to more closely approach the RROB value when they were provided with accuracy feedback and extensive practice. Older participants showed no evidence of making boundary adjustments in response to feedback or task practice, and they consistently used boundary separation values that produced accuracy levels that were near asymptote. Our results suggest that young participants attempt to balance speed and accuracy to achieve the most correct answers per unit time, whereas older participants attempt to minimize errors even if they must respond quite slowly to do so.To make good decisions, one must gather an appropriate amount of information before selecting an alternative. Gathering more information leads to more accurate but slower decisions, a situation referred to as the speed-accuracy tradeoff. Decision makers are faced with the dilemma of adopting a level of conservativeness that appropriately balances the speed and accuracy of their responding.Prior research shows that young and older adults differ in terms of how they balance speed and accuracy. Older adults make decisions slowly and avoid errors, whereas young adults decide more quickly and are more accepting of errors (Baron & Matilla, 1989;Hertzog, Vernon, & Rypma, 1993;Rabbitt, 1979;Salthouse, 1979;Smith & Brewer, 1985, 1995. Perhaps older adults are influenced by a lifetime of experience in which arriving at the correct answer is well worth the necessary deliberation. In this article, we explore the difference between young and older adults by examining the optimality of boundary settings using the diffusion model. As we explain in detail below, the diffusion model is a model of two-choice decision tasks that makes choices based on evidence accumulated over time (Ratcliff, 1978;Ratcliff & McKoon, 2008). One advantage of applying the model is that it accounts for speed-accuracy tradeoffs in terms of a single parameter representing response conservativeness: boundary separation. Boundary separation determines the amount of evidence that must accumulate in favor of a response alternative before a choice ...

Age-related differences in diffusion model boundary optimality with both trial-limited and time-limited tasks

“…For each participant, we found the RROB value in each difficulty condition with a search routine (see Starns & Ratcliff, 2010). The routine adjusted the boundary width, whereas all other parameters were held at the values produced in fits to data (with starting point coded as a proportion of the boundary width).…”

“…In the present experiment, we included another deviation from the experiments analyzed by Starns and Ratcliff (2010) that allowed for more fine-grained optimality analyses. Starns and Ratcliff used existing diffusion model experiments that included a number of conditions differing in task difficulty, but these conditions were mixed within blocks.…”

“…Therefore, RROB values provide an important reference point when considering how boundaries change across variables, populations, or tasks. Starns and Ratcliff (2010) found RROB values for a number of data sets and compared them to the participants' actual boundary settings. For both young and older participants, actual boundary values were consistently wider than the RROB values, but younger participants were closer to rewardrate optimality than were older participants.…”

“…The primary goal of the present experiment was to test the explanation for age differences offered by Starns and Ratcliff (2010)-that is, to determine whether age differences in boundary optimality simply reflect different task goals for young and older participants. We tested this account by including a condition in which both age groups were given the explicit goal of optimizing reward rate.…”

“…Recently, Starns and Ratcliff (2010) used optimality analyses to explore why older adults consistently set wider boundaries than young adults. For a given decision task, a single boundary value leads to the most correct answers per unit time, a metric that is often referred to as reward rate (Bogacz, Brown, Moehlis, Holmes, & Cohen, 2006;see Gold & Shadlen, 2002 for application to the animal literature).…”