New Perspectives in Computing the Point of Subjective Equality Using Rasch Models

To assess multiple psychometric characteristics of a new stroke outcome measure, the Stroke Impact Scale (SIS), using Rasch analysis, and to identify and remove misfitting items from the 8 domains that comprise the SIS. Secondary analysis of 3-month outcomes for the Glycine Antagonist in Neuroprotection (GAIN) Americas randomized stroke trial. A multicenter randomized trial performed in 132 centers in the United States and Canada. A total of 696 individuals with stroke who were community-dwelling and independent prior to acute stroke. Not applicable. Rasch analysis was performed using WINSTEPS, version 3.31, to evaluate 4 psychometric characteristics of the SIS: (1) unidimensionality or fit (the extent to which items measure a single construct), (2) targeting (the extent to which the items are of appropriate difficulty for the sample), (3) item difficulty (the ordering of items from least to most difficult to perform), and (4) separation (the extent to which the items distinguish distinct levels of functioning within the sample). (1) Within each domain, most of the items measured a single construct. Only 3 items misfit the constructs and were deleted ("add and subtract numbers," "get up from a chair," "feel emotionally connected") and 2 items ("handle money," "manage money") misfit the combined physical domain. These items were deleted to create SIS, version 3.0. (2) Overall, the items are well targeted to the sample. The physical and participation domains have a wide range of items that capture difficulties that most individuals with stroke experience in physical and role functions, while the memory, emotion, and communication domains include items that capture limitations in the most impaired patients. (3) The order of items from less to more difficult was clinically meaningful. (4) The individual physical domains differentiated at least 3 (high, average, low) levels of functioning and the composite physical domain differentiated more than 4 levels of functioning. However, because difficulties with communication, memory, and emotion were not as frequently reported and difficulties with hand function were more frequently reported, these domains only differentiated 2 (high, low) to 3 (high, average, low) strata of patients. Time from stroke onset to administration of the SIS had little effect on item functioning. Rasch analysis further established the validity of the SIS. The domains are unidimensional, the items have an excellent range of difficulty, and the domain scores differentiated patients into multiple strata. The activities of daily living/instrumental activities of daily living, mobility, strength, composite physical, and participation domains have the most robust psychometric characteristics. The composite physical domain is most able to discriminate difficulty in function in individuals after stroke, while the communication, memory, and emotion domain items only capture limitations in function in the more impaired groups of patients.

As research on sensation and perception has grown more sophisticated during the last century, new adaptive methodologies have been developed to increase efficiency and reliability of measurement. An experimental procedure is said to be adaptive if the physical characteristics of the stimuli on each trial are determined by the stimuli and responses that occurred in the previous trial or sequence of trials. In this paper, the general development of adaptive procedures is described, and three commonly used methods are reviewed. Typically, a threshold value is measured using these methods, and, in some cases, other characteristics of the psychometric function underlying perceptual performance, such as slope, may be developed. Results of simulations and experiments with human subjects are reviewed to evaluate the utility of these adaptive procedures and the special circumstances under which one might be superior to another.

Improvements in measuring thresholds, or points on a psychometric function, have advanced the field of psychophysics in the last 30 years. The arrival of laboratory computers allowed the introduction of adaptive procedures, where the presentation of the next stimulus depends on previous responses of the subject. Unfortunately, these procedures present themselves in a bewildering variety, though some of them differ only slightly. Even someone familiar with several methods cannot easily name the differences, or decide which method would be best suited for a particular application. This review tries to illuminate the historical background of adaptive procedures, explain their differences and similarities, and provide criteria for choosing among the various techniques.