A reason for caution: the identification of sudden gains may depend on the outcome measure used.

Sudden gains are rapid, substantial, and lasting symptom reductions between sessions which have been associated with better treatment outcomes. Frequent symptom measurements in treatment change research can be burdensome; ultra-brief scales can reduce this burden, increase completion rates, and enable the examination of a broader range of constructs. This study compared the timing, prevalence, and treatment outcomes associated with sudden gains during an internet-delivered treatment using standard and ultra-brief self-report measures of depression and anxiety. Existing data (N = 937) from a five-week internet-delivered treatment for depression and anxiety in university students was analysed. Anxiety symptoms were measured using the Generalized Anxiety Disorder 7-item Scale (GAD-7) and its brief version the GAD-2, and depression symptoms were measured using the Patient Health Questionnaire 9-item Scale (PHQ-9) and its brief version the PHQ-2. More sudden gains were identified by the PHQ-9 than the PHQ-2, and more by the GAD-2 than the GAD-7. Critically, the standard and ultra-brief measures identified different individuals as having experienced a sudden gain. Sudden gains were associated with higher baseline symptom severity. These results seem to raise important questions about the detection of sudden gains, and the comparability of findings across studies using different measures.