Characterization of asthma in adults: A comprehensive instrument

This study sought to identify the attributes that represent the character of asthma and to identify how these attributes might be measured and modeled. The theoretical framework incorporated a research-developed framework identifying seven concepts: Physiological Intensity, Somatic Vulnerability, Self-Management, Medication Management Intensity, Symptom lntensity, Functional Status, and Well Being. These themes guided the development of the instrument, the Asthma Outcome Index.

An initial pool of 74 items was generated. Content validity was supported by four content experts. Readability, comprehension, and completeness were assessed by physician, staff, and patient focus groups. The 74-item Asthma Outcome Index was pilot tested with a purposive sample of 50 adults with asthma. The instrument was revised in light of ongoing content expert evaluation, pilot participant feedback, and data analysis to yield a version with 85 items.

The 85-item instrument was tested on a purposive sample of 203 adults with asthma. Prior to analysis, items with item-to-scale correlations below 0.3 and at or above 0.7 were eliminated. Following this revisions, eight researcher-developed measurement scales were psychometrically tested for reliability and validity. Six of the eight scales, the "Symptom Intensity G Scale" ($\alpha$ = 0.757), The "Symptom Intensity B Scale" ($\alpha$ = 0.868), the "Management Intensity Scale" ($\alpha$ = 0.724), the "Functional Status Scale" ($\alpha$ = 0.765), the "Environmental-Impact Scale" ($\alpha$ = 0.744), and the "Somatic Vulnerability B Scale" ($\alpha$ = 0.785), were judged reliable using coefficient alpha and squared multiple correlation. Alpha correlation for the "Somatic Vulnerability A Scale" ($\alpha$ = 6312) and the "Medication Management Intensity Scale" ($\alpha$ = 0.673) was lower than the recommended by Nunnally (1978) for newly-developed scales. Validity of all measures was determined with confirmatory factor analysis using EQS 5.1 (Bentler & Wu, 1995) and found adequate under Bollen's (1989) definition of validity: all measurement variables were significantly linked to their hypothesized latent constructs. The latent constructs of the three factor model were Severity, Self-Management, and Illness Intensity.