The Use of External Anchors When Examining Differences in Scale Performance in Patient Experience Surveys.

Objectives: To present an example of using vignettes as an external anchor to assess measurement equivalence for patient experience measures.

Background: Evaluating measurement equivalence and differences in scale use is helpful for identifying disparities in patient experience based on patient surveys. External anchors, often in the form of scored vignettes, provide an attractive approach to examining differences in scale use but are not commonly used.

Methods: We analyzed a UK dataset based on the General Practice Patient Survey and a U.S. dataset based on the Consumer Assessment of Healthcare Providers and Systems Clinician and Group survey. A total of 560 White British and 560 Pakistani adults were recruited from various locations across England; 575 Asian American and 505 non-Hispanic White patients were recruited from an internet panel in the United States. Patient encounters and rated the quality of communication using 5 General Practice Patient Survey questions and 3 Consumer Assessment of Healthcare Providers and Systems Clinician and Group questions.

Results: Using an external anchor in both United States and UK data produced substantial evidence of differential item functioning (DIF). However, an "internal" DIF analysis (without an external anchor) produced little evidence of DIF.

Conclusions: Using an external anchor does not require the assumption made by internal methods that some items do not display between-group DIF. These assumptions may not hold for patient experience items if a single factor, such as extreme or negative response tendency, governs all items equally.