There are numerous benefits to answer-until-correct (AUC) approaches to multiple-choice testing, not the least of which is the straightforward allotment of partial credit. However, the benefits of granting partial credit can be tempered by the inevitable increase in test scores and by fears that such increases are further contaminated by a large random guessing component. We have measured the effects of using the immediate feedback assessment technique (IF-AT), a commercially available AUC response system, on the scores of a typical first-year chemistry multiple-choice test. We find that with a particular commonly used scoring scheme the test scores from IF-AT deployment are 6-7 percentage points higher than from Scantron deployment. This amount is less than that suggested by previous studies, where the mark increase was calculated in a purely post hoc manner and thus neglected affective changes of students' behavior associated with the IF-AT technique. Furthermore, we have strong evidence that partial credit is awarded in a highly rational manner in accordance with the students' level of understanding.