Reliability and agreement in the use of four- and six-point ordinal scales for the assessment of erythema in digital images of canine skin

BACKGROUND: Grading of erythema in clinical practice is a subjective assessment that cannot be confirmed using a definitive test; nevertheless, erythema scores are typically measured in clinical trials assessing the response to treatment interventions. Most commonly, ordinal scales are used for this purpose, but the optimal number of categories in such scales has not been determined. HYPOTHESIS/OBJECTIVES: This study aimed to compare the reliability and agreement of a four-point and a six-point ordinal scale for the assessment of erythema in digital images of canine skin. METHODS: Fifteen digital images showing varying degrees of erythema were assessed by specialist dermatologists and laypeople, using either the four-point or the six-point scale. Reliability between the raters was assessed using intraclass correlation coefficients and Cronbach's α. Agreement was assessed using the variation ratio (the percentage of respondents who chose the mode, the most common answer). Intraobserver variability was assessed by comparing the results of two grading sessions, at least 6 weeks apart. RESULTS: Both scales demonstrated high reliability, with intraclass correlation coefficient values and Cronbach's α above 0.99. However, the four-point scale demonstrated significantly superior agreement, with variation ratios for the four-point scale averaging 74.8%, compared with 56.2% for the six-point scale. Intraobserver consistency for the four-point scale was very high. CONCLUSIONS AND CLINICAL IMPORTANCE: Although both scales demonstrated high reliability, the four-point scale was superior in terms of agreement. For the assessment of erythema in clinical trials, a four-point ordinal scale is recommended.

Peter B. Hill