Abstract
The Alzheimer´s Disease Knowledge Scale (ADKS) is a 30-item instrument to assess what people know about AD covering risk factors, diagnosis, symptoms, course, functional impact, caregiving, and management. The aim of this study was to explore scale item characteristics with item response theory (IRT) procedures. A noninterventional, web-based study was conducted among employees of Roche Spain. We compared ADKS scores between participants with and without relatives diagnosed with AD. A nonparametric IRT procedure, Mokken analysis, was performed to explore its underlying structure. A parametric IRT analysis was then used to compare the fitting of two different models for dichotomous items, the one-parameter logistic model (1PL) that assumes the items show different difficulty but share the same discrimination parameter, and the two-parameter logistic model (2PL) that assumes the items show different difficulty and discrimination parameters. A total of 447 participants completed the ADKS. The mean ADKS score was 21.2 (SD 2.8). Participants with a first-degree relative with AD (N=42, 9%) scored higher (mean difference 1.06, p-value = 0.02; standardized mean difference 0.38, indicating a small to moderate effect). Mokken analysis showed that most items (22/30) did not fit to any scale and could be considered scale independent. Two items fitted a knowledge domain related with depression, another two items were related with AD prevention and recovery, and four items fitted a risk factors domain. The likelihood ratio test (LRT) between 1PL and 2PL models was significant (LRT 198.91, df 29, p-value < 0.0001), favoring 2PL. The overall ADKS score distinguishes knowledge about dementia among people with or without AD relatives. IRT analyses suggest an independence among items that is highly relevant for a general knowledge scale. In summary, the 30-item ADKS, as originally reported, could be used to assess the public´s knowledge and to develop and guide educational interventions if needed.
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