Modeling Item Response Times With a Two-State Mixture Model: A New Method of Measuring Speededness


DEBORAH L. SCHNIPKE is Research Scientist, Law School Admission Council, 661 Penn St., Newtown, PA 18940, Degrees: BS, Bowling Green State University. MA, Phi), Johns Hopkins University. Specializations: computer-based testing, psychometrics

DAVID J. SCRAMS is a graduate student, Department of Psychology, Johns Hopkins University, 3400 North Charles St., Baltimore, MD 21218; Degrees: BA, California State University, Bakersfield; MA, Johns Hopkins University. Specializations: mathematical models, cognitive psychology.


Speededness refers to the extent to which time limits affect examinees'test performance, and it is often measured by calculating the proportion of examinees who do not reach a certain percentage of test items. However, when tests are number-right scored (i.e., no points are subtracted for incorrect responses), examinees are likely to rapidly guess on items rather than leave them blank. Therefore, this traditional measure of speededness probably underestimates the true amount of speededness on such tests. A more accurate assessment of speededness should also reflect the tendency of examinees to rapidly guess on items as time expires. This rapid-guessing component of speededness can be estimated by modeling response times with a two-state mixture model, as demonstrated with data from a computer- administered reasoning test. Taking into account the combined effect of unreached items and rapid guessing provides a more complete measure of speededness than has previously been available.