A total of 421 papers were identified. An explicit theoretical definition was found in 33 (7.8%) papers. Data collection procedure was mentioned in 248 (58.9%) and 104 (24.7%) papers described how data was converted into ethnic groups. The operationalizations of ethnicity have remained almost unchanged during the 15-year review period. Generally, the ethnic variable was incompletely reported. Confusion regarding which individual or social characteristics ethnicity refers to makes the research findings of limited value in clinical settings, and may continue to create misunderstanding about the effect of ethnicity in clinical contexts.