Objective: We have compared two philosophically different methods for selecting items for a disease-specific quality of life questionnaire. The impact method selects items that are most frequently perceived as important by patients whereas the psychometric method (factor analysis) selects items primarily according to their relationships with one another.
Patients: 150 adults with symptomatic asthma and a wide range of disease severity were enrolled from asthma clinics and notices in the local media.
Study design: From a list of 152 items that are potentially troublesome to patients with asthma, the patients identified those items they had experienced in the previous year and scored the importance of each on a five-point scale. For the impact method, items that were identified most frequently and that scored the highest were included in the final instrument. For the psychometric method, factor analysis was performed after highly skewed items had been removed. Items with high factor loading were included in the final instrument.
Results: The impact method resulted in a 32-item instrument and psychometric analysis in one with 36 items. Twenty items were common to both instruments. The psychometric approach discarded the highest impact emotional function and environmental items and included in their place lower impact items mainly associated with fatigue.
Conclusions: Although some items were the same for both methods, there were also some important differences. Different approaches to item reduction led to appreciably different instruments.