Psoriasis Disability Index
Title: The psychometric properties of the psoriasis
disability index in United States patients.
Authors: 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
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.
: Psychometrics is the branch of psychology
that deals with the use of quantitative measurements of psychological
variables, such as intelligence, aptitude, and personality traits.