Fundamentals of measurement in health care research

This article discusses levels of measurement and their application to research and practice in health care. The concept of levels of measurement was codified in a seminal article by S. S. Stevens in 1946 that defined four levels of measurement: nominal scales, which label and classify cases (objects and individuals) and assign them to categories; ordinal scales, which rank cases on some attribute; interval scales, which have equal intervals for measuring attributes; and ratio scales, which have equal intervals and a natural zero point. The rules that apply to each level of measurement are presented and the mathematical operations that can be performed on them are explained. The similarities and differences among the four types of scales are discussed and examples of their use in health care and other contexts are described.