Background: The Strengths and Difficulties Added Value Score (SDQ AVS) uses a large epidemiological study to predict follow-up parental SDQ scores for the evaluation of routine outcomes. Method: We tested the prediction of the SDQ AVS derived from a national population survey separately on scores for the waiting list control and intervention groups in a randomised controlled trial. If the SDQ AVS is to be clinically useful, it needs to function as expected across different populations. Results: In the control arm, the SDQ AVS predicted an effect size of 0.15 (95% CI −0.01–0.30) compared to an expected effect size of 0, as the children in this arm received no treatment. In the experimental arm, the SDQ AVS predicted an effect size of 0.62 (95% CI 0.42–0.83) compared to the study effect size of 0.53. Change scores overestimated the effect size in both arms (control 0.50 95% CI 0.34–0.66, intervention 0.85 95% CI 0.66–1.04). Conclusion: Our findings suggest that the SDQ AVS adjusts for spontaneous improvement, regression to the mean and attenuation.