To the Editor: Impulsivity, a psychological construct comprising both motor and cognitive factors, is thought to underlie important interindividual differences in health and disease (1). In particular, reflection impulsivity, which refers to the tendency to gather and evaluate information before decision making (2), has been implicated in many psychiatric and neurological disorders (3, 4, 5). One of the standard tasks for measuring reflection impulsivity in healthy and clinical populations is the Information Sampling Task (IST), designed by Clark et al. (3) and included in the widely used Cambridge Neuropsychological Test Automated Battery (CANTAB) (6). In this CANTAB version of the IST, participants sample a variable amount of information about an uncertain outcome before making a decision. The amount of information sampled before the decision gives a measure of participants’ reflection impulsivity. In this correspondence, we show that the calculation of the IST’s main outcome measure, P(correct), is based on incorrect statistical inference, resulting in systematic overestimation of participants’ reflection impulsivity and potentially inflated type II error rates. This might affect the results of numerous recent psychopharmacological, neuropsychological, and psychiatric publications that have used the IST (4, 5, 7).