Studies of sustained attention using the Continuous Performance Test (CPT) often incorporate effortful processing conditions and different response measures. Difficulty of attentional effort and interrelationships among response measures, however, have not been well established particularly with normal populations. This study investigated three CPT tasks which varied in interstimulus interval and target frequency in 107 nonpatient adults. This report also examined five response measures (d′, a measure of discriminability; In(β), a measure of response bias; reaction time; and omission and commission errors). Findings indicated that a high target frequency condition was more effortful than a short interstimulus interval condition on all response measures. Further, d′, omission errors, and commission errors differentiated the short interstimulus interval condition from baseline, but reaction time and In(β) did not. Participants with faster reaction times in the high target frequency condition had fewer omission errors but more commission errors; these relationships may have been mediated by response bias. This suggested that response measures were interrelated but nonredundant, and that these relationships were influenced by attentional effort and response bias.