The inter-item standard deviation (ISD): An index that discriminates between conscientious and random responders

Although random responding is prevalent and increases Type II errors, most psychologists avoid trying to identify it because the means to do so are extremely limited. We propose the inter-item standard deviation (ISD), a statistical index of response variance, is suited for this task. We hypothesized that random responders produce large ISDs because they respond to items all over a measure's response range, whereas conscientious responders produce small ISDs because they respond to items more consistently. We administered a questionnaire containing the NEO-FFI-3 and an embedded validity scale to 134 university students. Another 134 responders were created using a random number generator. For all 268 responders, the ISD was calculated for each of the NEO-FFI-3′s five subscales and an aggregated ISD was calculated by averaging the five ISD indexes. Results showed that (1) random responders produce significantly larger ISDs than conscientious responders, (2) the ISDs were strongly correlated with the embedded validity scale and with one another, and (3) the ISDs correctly identified responders with greater than 80% classification accuracy. The mean ISD yielded greater than 95% classification accuracy. This study shows that responders can be identified by quantifying inter-item response variance.