Scientists and clinicians use measurement as a way of understanding, evaluating and differentiating characteristics of people and objects. Measurement provides a mechanism for achieving a degree of precision in this understanding, so that we can describe physical or behavioral characteristics according to their quantity, degree, capacity or quality.1 We can document that a patient's shoulder can flex to 75 degrees, rather than say motion is "limited," or indicate that the air temperature is 95° F, rather than just "hot." This ability helps us communicate information in objective terms, giving us a common sense of "how much" or "how little" without ambiguous interpretation. Principles of measurement, therefore, are basic to our ability to describe phenomena, demonstrate change or relationship, and to communicate this information to others.
Measurement is used as a basis for making decisions or drawing conclusions in several ways. At its most basic, measurement is used to describe the quality or quantity of an existing variable, such as the measurement of intelligence, attitude, range of motion or muscle strength. We can also use measurement to make absolute decisions based on a criterion or standard of performance, such as the requirement that a student achieve at least a grade of C to pass a course or that a certain degree of spinal curvature be present to indicate a diagnosis of scoliosis. We use measurement as a basis for choosing between two courses of action. In this sense a clinician might decide to implement one treatment approach over another based on the results of a comparative research study. Clinicians use measurement as a means of evaluating a patient's condition and response to treatment; that is, we measure change or progress. We also use measurements to compare and discriminate between individuals or groups. For instance, a test can be used to distinguish between children who do and do not have learning disabilities or between different types of learning disabilities. Finally, measurement allows us to draw conclusions about the predictive relationship between variables. We might use grades on a college entrance examination to predict a student's ability to succeed in an academic program. We can measure the functional status of an elderly patient to determine the level of assistance that will be required when the patient returns home. There are virtually no decisions or clinical actions that are independent of some type of measurement.
Measurement has been defined as the process of assigning numerals to variables to represent quantities of characteristics according to certain rules.2 The purpose of this chapter is to explore this definition as it is applied to clinical research. In doing so, we consider several aspects of measurement theory and discuss how these relate to measurement, analysis and interpretation of clinical variables.
QUANTIFICATION AND MEASUREMENT
The first part of the definition of measurement emphasizes the process of assigning numerals to variables. A numeral is ...