Introduction

The World Health Organization defines an outcome measure as a “change in the health of an individual, group of people, or population that is attributable to an intervention or a series of interventions.”¹ Outcome measurement is becoming increasingly important in an effort to document the value of interventions to both patients and society. Harvard Business School professor and healthcare policy expert Michael Porter has proposed a framework that defines value as health outcomes achieved relative to the costs incurred.² Outcome measures are critical to both clinical research and public health, because this is the primary driver influencing one’s ability to answer important questions in a reliable manner. Outcome measures serve as the target that both researchers and healthcare organizations or governments monitor or attempt to modulate in an effort to improve the quality and/or cost of care. The past 30 years have seen a rise in interest in the measurement of the outcomes of medical care, to the extent that an “outcomes movement” has been described and been labelled “the third revolution in healthcare.”³ Types of outcome measures can be seen, broadly, as biophysical measures, like morbidity and mortality, or patient‐based measures that incorporate a patient’s subjective experience of illness.

Top three questions

1. What is an outcome measure?
   
2. What properties of outcome measures do I have to know?
   
3. How should I choose an outcome measure?

Question 1: What is an outcome measure?

An outcome measure is a measure of the health of an individual, group of people, or population. There are several broad categories of outcome measure:

1. Biophysical outcome measures: these are objective health measures. Some common examples include morbidity, mortality, complication rate, and quality of reduction.
   
2. Patient‐based measures: these outcome measures incorporate a patient’s subjective experience of illness. These may be generic or disease/joint‐specific.
   
   
   
   1. Generic outcome measures are not specific to any one disease or anatomic location. They can be used to compare across or within specific pathologic conditions. These have the potential advantage of being more able to measure downstream consequences of a treatment or condition that permits comparison to other unrelated conditions. They also may measure the side effects of complications of a treatment or condition that occur in a different anatomic location. Two common examples of a generic patient‐reported outcome measure are the Short Form 36 (SF‐36) and the Patient‐Reported Outcomes Measurement Information System (PROMIS) Physical Function. Preference‐weighted outcome measures are a subset of generic outcome measures. These outcome measures weight items or dimensions differently to account for how people value a health state, rather than assigning equal weight to each dimension or item included in the outcome measure. Measuring preference‐weighted generic outcome measures is becoming increasingly important in an effort to optimally distribute resources in a resource‐constrained environment. The most commonly used preference‐weighted generic outcome measures are the EuroQol five‐dimensional questionnaire (EQ5D) and the six‐dimensional health state short form (SF6D).
      
   2. Disease‐ or joint‐specific outcome measures may be more sensitive measures of the specific disease or joint being assessed. For example, joint‐specific outcome measures have been shown to be more sensitive to arthroplasty procedures compared to generic outcome measures.^4–7 Common joint‐specific outcome measures in orthopedic surgery include the Western Ontario and McMaster Universities Arthritis (WOMAC) Index, Harris Hip Score (HHS) and the Hip Disability and Osteoarthritis Outcome Score (HOOS) Knee Injury and Osteoarthritis Outcome Score (KOOS).

Question 2: What properties of outcome measures do I have to know?

There are several properties and attributes that have to be considered when choosing appropriate outcome measures.