extended · article · 2007

Why Measurement Matters for Measuring Patient Vision Outcomes

Trudy Mallinson

Optometrists, by definition, care deeply about measurement. This brief review article considers the essential features of measurement that make many optometric instruments so useful and how patient-centered survey instruments such as vision-related quality of life questionnaires, can be analyzed using contemporary psychometric methods, so that they also conform to these essential features of measurement. These features include unidimensionality, hierarchical order, and equal interval scaling. Optometrists demand these features because they need to make meaningful comparisons both between patients and over time. Questionnaires about visual function or health-related quality of life, typically involve a series of rating scale type items that are added up to produce a total raw score. Yet total raw scores, which are ordinal, do not exhibit the essential properties of measurement. The Rasch ...