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This study investigated the effects of polytomous item response theory models and item wording direction on estimated latent trait scores. The original version of the 10-item Perceived Stress Scale (Cohen, Kamarck, & Mermelstein, 1983) was administered to the sample, along with 10 matched, reverse-worded items. Model fit indices, test information, standard errors, and trait scores from three polytomous IRT models were compared for responses to the set of 20 items (10 positive and 10 negative items), and the subsets of 10 positive, 10 negative, 10 original items, and 10 reverse-worded items. The nominal response model may provide more maximal for those with a trait score in the middle of the distribution, but the graded response model provides more information (and smaller standard errors) of trait scores across a larger span of the scale. Scores were highly correlated, but those from subsets of positively worded and negatively worded items were only moderately correlated. The use of scales with all positively worded items was statistically more favorable, but further research is necessary to determine favorability in practice. The results have implications for researchers and practitioners using psychological scales or developing scales with polytomous responses for diagnostic purposes.

The generalized partial credit model (GPCM) is often used for polytomous data; however, the nominal response model (NRM) allows for the investigation of how adjacent categories may discriminate differently when items are positively or negatively worded. Ten items from three different self-reported scales were used (anxiety, depression, and perceived stress), and authors wrote an additional item worded in the opposite direction to pair with each original item. Sets of the original and reverse-worded items were administered, and responses were analyzed using the two models. The NRM fit significantly better than the GPCM, and it was able to detect category responses that may not function well. Positively worded items tended to be more discriminating than negatively worded items. For the depression scale, category boundary locations tended to have a larger range for the positively worded items than for the negatively worded items from both models. Some pairs of items functioned comparably when reverse-worded, but others did not. If an examinee responds in an extreme category to an item, the same examinee is not necessarily likely to respond in an extreme category at the opposite end of the rating scale to a similar item worded in the opposite direction. Results of this study may support the use of scales composed of items worded in the same direction, and particularly in the positive direction.

This chapter considers the types of measures used in rehabilitation and health settings, including their alignment with the major health and disease diagnostic and classification systems. The chapter provides an overview of the needs that are served by rehabilitation and health assessments and highlights the increasing importance of measures that yield data to guide decisions to support functioning across a broad range of life domains. It makes the case for prospective rehabilitation and health assessments that predict more reliably future functioning in preferred activities taking into account personal and environment interactions. Environmental factors include the context for both achieved and aspired participation and for which rehabilitation and health supports are intended. The future of rehabilitation and health assessments is tied to their ability to provide data that translate directly into needed support services for maximal participation by people living with debilitating health conditions.

Assessment is critical to rehabilitation practice and research, and self-reports are a commonly used form of assessment. This study examines a gender effect according to item wording on the Perceived Stress Scale for adults with multiple sclerosis. Past studies have demonstrated two-factor solutions on this scale and other scales measuring stress-related constructs with factor loadings being determined by item wording. Moreover, women have typically scored higher on these measured constructs. However, a literature review reveals that this gender difference often manifests only on the factor composed of negatively worded items. This study extends this line of research by examining gender differences on the Perceived Stress Scale on the negatively worded items at both the item and bundle levels after controlling for responses on the positively worded items. Implications of this study on the field of rehabilitation are discussed.

Measurement is critical to Rehabilitation practice and research. To provide effective services and to have valid research results, it is imperative to have valid and reliable assessment tools. Differential Item Functioning (DIF) is a statistical procedure which better enables us to assess both the validity and the results of measurement instruments. As such, it is a methodology that can be very beneficial to the field of Rehabilitation. The purposes of this article are to (a) explain the fundamental principles and components of DIF, (b) discuss the practical implications of DIF for the Rehabilitation discipline, and (c) provide an example of DIF implementation and interpretation of results.