Item Response Modeling of Presence-Severity Items: Application to Measurement of Patient-Reported Outcomes
Ying Liu and Jay Verkuilen
The Presence-Severity (P-S) format refers to a compound item structure which has two parts：First, the Presence part, is used to check whether the respondent experiences the particular event in question. If the answer of the Presence part is yes, then a follow-up question, the Severity part is asked, often about the frequency, density, severity, or impact of the event. Although the P-S format is widely used, it has not been subject to much careful analysis to help users decide key questions related to its psychometric feature.
The NRM estimated using maximum marginal likelihood (MML) methods is used for the analysis using a special stratified contingency table with fused cells. The two parts items then combine into one compound item. The common 2 * 2 table are transformed to a 1 * 3 table to accurately describe the real situation.
The data is from the Memorial Symptom Assessment Scale–Short Form (MSAS-SF)
Questions:
It is more convenient to compose a questionnaire using compound items?
Since ak is the slope parameter, then will the use a1/ ao instead (a1-a2) more suitable parameter for judgement?