When we began to develop Skindex, we were greatly informed by Andrew Finlay’s previous work on measuring disability from skin disease 1,2
. Our goal was to develop an instrument to measure comprehensively the effects of skin disease on health-related quality of life, and we specifically designed the instrument be able to discriminate between patients with different effects, as well as to detect changes in patients over time 3
. We followed an incremental strategy that began with an hypothesis: based on our literature review of previous clinical and psychological studies as well as substantial input from patients and clinicians, we constructed a comprehensive conceptual framework for the ways in which we hypothesized skin diseases affected patients. We composed survey items to measure all domains in the framework, and then we tested our hypothesis by examining the validity of the items in a series of psychometric tests using the responses of a large sample of patients.
Original Conceptual Framework
We proposed that skin diseases affect patients in either psychosocial or physical ways. We suggested that psychosocial effects could be cognitive (beliefs about self or others), social, or emotional. Subdimensions of emotional effects include depression, fear, embarrassment, and anger. Physical effects are either discomfort or limitations in physical functioning. This original hypothesized framework is depicted in .
Original hypothesized conceptual framework for the effects of skin disease on the quality of life of affected patients.
Item composition and prototype Skindex
Our team consisted of two psychometricians, and, using conventional principles, we composed 65 items to assess the dimensions in the conceptual framework. We pilot-tested this draft survey and changed or deleted ambiguous and redundant items, which left a 61-item prototype version of Skindex. The measurement properties of this trial version were tested in a series of studies that demonstrated it to be reliable and to have substantial evidence of validity as a measure of the effects of skin disease on quality of life 4
Refinement into Skindex-29
We wanted to improve the ability of the prototype Skindex to discriminate among patients with likely different degrees of quality-of-life effect, as well as to be more sensitive to even modest changes in patients’ experiences over time. Also, if possible, we wanted to shorten the instrument to make it more useful in research and clinical settings. To accomplish these goals, we assessed the performance of each item, using not only qualitative judgments but also based on a priori criteria for suboptimal item performance, including reproducibility, discriminant validity, complexity, ambiguity, response distribution, and item-total correlation. We analyzed the factors, or themes, that explained the variability in responses to the psychometrically most sound items, which permitted us to test and refine our theorized model for the effects of skin disease on quality of life. Finally, we composed new items that we judged would improve discriminative and evaluative capability of the instrument.
This sequential process generated a refined conceptual framework; we now propose that the effects of skin disease on quality of life can be understood in three domains: Symptoms, Emotions, and Functioning (). The analyses yielded a 29-item version of Skindex that remained reliable and valid, but that had reduced respondent burden, and improved discriminative and evaluative capability 5
Refined conceptual framework for the effects of skin disease on the quality of life of affected patients.
Skindex-29 inquires about how often (Never, Rarely, Sometimes, Often, All the time) during the previous four weeks the patient experienced the effect described in each item. Seven items address the Symptoms domain, ten items the Emotional domain, and twelve items the Functioning domain. All responses are transformed to a linear scale of 100, varying from 0 (no effect) to 100 (effect experienced all the time). Skindex scores are reported as three scale scores, corresponding to the three domains; a scale score is the average of a patient’s responses to items in a given domain.
Construction of Skindex-16
Longitudinal research studies often require waves of data collection with lengthy survey instruments to assess multiple aspects of patients’ experience. We wanted to develop a version of Skindex that would remain accurate and responsive as a measure of skin-related quality of life, but that would be brief (contained on one page). Also, we wanted to assess not only how often patients have a particular experience, but how much they are bothered
by it. Thus, we used Skindex-29 as the substrate for a series of studies that developed a different, single-page version, Skindex-16. We used item analyses similar to those described above, to select only those items that performed well according to our criteria, and eliminated items to which a majority of patients responded ‘Never’. We also composed new items to address aspects of quality of life that patients mentioned often in their qualitative responses but that had not been addressed in Skindex-29. This process generated a new instrument, which fits on a single page. The header inquires “During the past x weeks, how often have you been bothered by
The response choices are on continuous bipolar scale with seven boxes anchored by the words “Never Bothered” and “Álways Bothered”
at each end. As with the parent instrument, scores vary from 0 (no effect) to 100 (effect experienced all the time), and responses are aggregated in Symptoms (four items), Emotions (seven items), and Functioning scales (five items). We tested the performance of this new instrument, Skindex-16, in over 500 patients, and determined that it was reliable, retained substantial evidence of validity, and was responsive to clinical change 6