The foundational measurement concepts described in preceding chapters create a context for understanding how we can use evidence to interpret scores from appropriate measurement tools. Because many aspects of health reflect intangible constructs, they require data that must be converted to a scale that will indicate “how much” of a health factor is present in an individual.
The purpose of this chapter is to describe different types of scales used in questionnaires to assess health. These may be self-report inventories or performance-based instruments. This discussion will focus on validity of four types of scales, including summative, visual analog, and cumulative scales, and the Rasch measurement model. Whether you are trying to use or modify established instruments or construct a new tool, an understanding of scale properties is essential for interpretation of scores and application to clinical decision-making.
Understanding the Construct
The first essential step in choosing or developing a health scale is understanding the construct being measured. The items in a scale must reflect the underlying latent trait and the dimensions that define it (see Chapter 10). Scales require substantial measurement scrutiny to establish their reliability and validity, as we strive to confirm utility with different populations and under different clinical conditions.
An important characteristic of a scale’s validity is its focus on a common dimension of the trait being measured. This requires a clear theoretical foundation, including behavioral expectations when the trait is present to different degrees, and an understanding of distinct components of the construct.
Consider the study of pain, which can be manifested in several dimensions, including differences between acute and chronic pain, and potential biopsychosocial impacts related to symptoms, how it affects movement, or its impact on quality of life.1 For example, the McGill Pain Questionnaire incorporates pain ratings as well qualitative descriptors, based on Melzack’s theory of pain reflecting sensory, affective, and evaluative dimensions of pain.2 It focuses on various aspects of pain, such as intensity, type of pain, and duration. In contrast, the Roland Morris Disability Questionnaire (RMDQ) was designed to assess physical disability due to low back pain.3 Therefore, the framework for the RMDQ is based on how back pain influences daily life, not on a description of the pain itself.
For many clinical variables, a single indicator, such as blood pressure or hematocrit, can provide relevant information regarding a specific aspect of health. These tests are typically interpreted with reference to a criterion that defines a normal range. These values play an important role in identifying disease or indicating appropriate treatment.
As the concept of health has broadened, however, single indicators are not sufficient to reflect the dimensions of important clinical traits. ...