Does Inclusion of Interactions Result in Higher Precision of Estimated Health State Values?
BACKGROUND
Most preference-based instruments producing overall values for health states are devised on the simplifying assumption that the overall effect of distinct health-related quality of life domains (attributes) of the instrument equals the sum of the attributes. Nevertheless, health attributes are often inter-related and depend on each other.


OBJECTIVES
To investigate whether inclusion of second-order interactions in the three-level EuroQol five-dimensional questionnaire (EQ-5D-3L) value function would result in better fit and lead to different health state values than a model with main effects only.


METHODS
Using an efficient design, 400 pairs of EQ-5D-3L health states were generated in a pairwise choice format. We analyzed responses of 4000 people from the general population using a conditional logit model, and we tested goodness of fit using pseudo R2, Akaike information criterion, differences in log-likelihood, and likelihood ratio. We compared accuracies of models' predictions based on root mean square error and mean absolute error.


RESULTS
The interaction-effects model showed systematically lower values than the main-effects model. Inclusion of interactions resulted only in a slightly better model fit. Interactions comprising mobility and self-care were the most salient.


CONCLUSIONS
For the EQ-5D-3L, a value function based on interactions produces systematically lower values than a main-effects model, meaning that the effect of two or more health problems combined is stronger than the sum of the individual main effects.
['Nicolet|Anna|A|', 'Groothuis-Oudshoorn|Catharina G M|CGM|', 'Krabbe|Paul F M|PFM|']
D000203:Activities of Daily Living D000293:Adolescent D000328:Adult D000368:Aged D002755:Choice Behavior D005260:Female D006304:Health Status D006801:Humans D008297:Male D008875:Middle Aged D051346:Mobility Limitation D008954:Models, Biological D015233:Models, Statistical D011788:Quality of Life D019057:Quality-Adjusted Life Years D012107:Research Design D012648:Self Care D011795:Surveys and Questionnaires D055815:Young Adult
values second-order interactions EQ-5D-3L main effects discrete choice