Psoriasis Disability Index

Title: The psychometric properties of the psoriasis disability index in United States patients.
: Nijsten T, et al.
Publication: J Invest Dermatol. 2005 Oct;125(4):665-72.

Although it is only used in the United Kingdom, the Psoriasis Disability Index or PDI is commonly used to measure the impact of psoriasis on patients.

The authors wanted to use PDI as a psychometric test to evaluate 1196 patients in the United States. They discovered that the PDI was internally consistent and reliable (by evaluating a statistical measure called the Cronbach alpha coefficient). When they used global questions about psoriasis’ burden and self-assessment of the extent of the disease, they found that the PDI has good validity.

However, they discovered a large floor effect (the results piled up at the low end) and suboptimal statistical distribution of the results, which suggested that the PDI was not sensitive enough to measure mild functional limitations.

Using a statistical analysis technique called the Rasch analysis, the authors found that the PDI appeared to measure multiple things, thus making efforts to boil it down to a single number questionable.

The authors concluded that PDI was not an optimal test for measuring psoriasis’ effect in the US population.

Editor’s Note: Psychometrics is the branch of psychology that deals with the use of quantitative measurements of psychological variables, such as intelligence, aptitude, and personality traits.

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