Increase of Uncertainty in Summed-Score-Based Scoring in Non-Rasch IRT

Summed-score (SS)-based scoring in non-Rasch IRT allows for pencil-and-paper administration and is used in the Patient-Reported Outcomes Measurement Information System (PROMIS) alongside response-pattern-based scoring. However, this convenience comes with an increase in uncertainty (the increase) associated with SS scoring. The increase can be quantified through the relationship between Bayesian SS and RP scoring. Given an SS of s, the SS posterior is a weighted sum of RP posteriors, with weights representing the marginal probabilities of RPs. From this mixture, the SS score (SS posterior mean) is a weighted sum of RP posterior means, and its uncertainty (variance of the SS posterior) is decomposed into the uncertainty of RP scoring (the weighted sum of RP posterior variances) and the increase (variance of RP posterior means). Without quantifying the increase, PROMIS recommends RP scoring for greater accuracy, suggesting SS scoring as a second option. Using variance decomposition, we quantified the increases for two short forms (SFs). In one, the increase is very small, making SS scoring as accurate as RP scoring, while in the other, the increase is large, indicating SS scoring may not be a viable second option. The increase varies widely, influencing scoring decisions, and should be reported for each SF when SS scoring is used.