Linking scores from multiple health outcome instruments.

To the extent that outcomes of health assessment instruments are to be used interchangeably, the summary scores based on these outcomes need to be equated or made comparable. If the summary scores of different health assessment instruments are not equated, inferences based on them could be flawed. Ideally, summary scores would be comparable because of careful instrument design. In practice, that rarely happens. Statistical intervention is usually needed. This article addresses key questions associated with the linking of summary scores of health outcomes. What is meant by outcome linking and equating? How does equating differ from other types of linking? What common data collection designs are used to capture data for outcomes linking? What are some of the standard statistical procedures used to link outcomes directly? What assumptions do they make? What role does IRT play in linking outcomes? What assumptions do IRT methods make? This article makes a distinction between direct statistical adjustments of summary score distributions, and indirect procedures based on psychometric models of items or questions.