title: The impact of events scale: a comparison of frequency versus severity approaches to measuring cancer-specific distress.
abstract: OBJECTIVE
  The Impact of Events Scale (IES) is one of the most widely used measures of event-specific distress. The IES assesses the frequency with which respondents experience intrusive thoughts and avoidant behaviors over the past week. Our aim is to demonstrate the benefit of a severity-based measurement approach of the IES compared with a frequency-based measurement approach.
  
  
  METHODS
  A mixed group of post-treatment cancer survivors (N = 325; M = 31.8 years old) completed measures assessing quality of life (Functional Assessment of Cancer Therapy-General), psychological adjustment (Mental Health Inventory), and cancer-related distress (IES). The IES was keyed to the cancer experience and administered with standard (frequency) and modified (severity) response options.
  
  
  RESULTS
  Classical reliability analyses and bifactor modeling were conducted on both versions of the IES. Reliability estimates suggest that the IES severity items were more highly intercorrelated than the IES frequency items. Both versions of the IES were highly correlated (r = 0.82), showing the presence of a dominant general factor. Bifactor modeling suggested that the severity items generally provided higher levels of discrimination than the frequency items. Validity correlations with the Functional Assessment of Cancer Therapy-General and Mental Health Inventory demonstrated that the IES severity performed as good as or better than the IES frequency.
  
  
  CONCLUSIONS
  Given the high correlations and similarity in content, the IES severity items largely assess the same construct as the IES frequency items. However, IES severity items generally showed improved psychometric properties and similar or higher correlations with quality of life and psychological adjustment. The IES severity approach appears to be a more informative method for assessing cancer-specific distress.
authors.affiliation: Department of Medical Social Sciences, Northwestern University Feinberg School of Medicine, Chicago, IL, USA.|The Robert H. Lurie Comprehensive Cancer Center of Northwestern University, Chicago, IL, USA.
authors.forename: John M
authors.initials: JM
authors.lastname: Salsman
authors.affiliation: Department of Medical Social Sciences, Northwestern University Feinberg School of Medicine, Chicago, IL, USA.
authors.forename: Benjamin D
authors.initials: BD
authors.lastname: Schalet
authors.affiliation: Department of Behavioral Science, University of Kentucky, Lexington, KY, USA.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Andrykowski
authors.affiliation: Department of Medical Social Sciences, Northwestern University Feinberg School of Medicine, Chicago, IL, USA.|The Robert H. Lurie Comprehensive Cancer Center of Northwestern University, Chicago, IL, USA.
authors.forename: David
authors.initials: D
authors.lastname: Cella
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008297:Male; D009369:Neoplasms / Q000523:psychology*; D011569:Psychiatric Status Rating Scales; D011594:Psychometrics; D011788:Quality of Life / Q000523:psychology; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D013315:Stress, Psychological / Q000175:diagnosis*
keywords: cancer; item response theory; measurement; oncology; psychosocial; trauma