In recent years, change scores obtained under neutral conditions and under faking-inducing conditions have become one of the main alternatives for operationalizing faking. A pending issue regarding these measures is the relevance of individual differences under similar conditions of pressure. This study proposes a simple approach based on the classic test theory that allows the issue to be rigorously assessed. The approach, from which three indices are derived, is based on a pre-test post-test design with a control group, and models the amount of change as an individual parameter. The proposal is applied to an empirical study in personality, and some interesting initial results are obtained.