Impact of an Event Scale

Abstract

The Impact of Event Scale (IES) is a psychological assessment tool consisting of a short set of self-report items designed to measure the intensity of subjective distress an individual associates with a specific stressful or traumatic event. Developed in 1979 by Horowitz, Wilner, and Alvarez, the original 15-item scale focuses on symptoms of intrusion and avoidance experienced during the preceding seven days. The IES is highly effective for clinical screening and research, capable of identifying both general stress reactions and symptoms severe enough to indicate probable Post Traumatic Stress Disorder (PTSD). A revised, 22-item version (IES-R) was later introduced in 1997 by Weiss and Marmar to expand the measurement to include symptoms of hyperarousal.

Keywords

Impact of Event Scale, IES, IES-R, Trauma Assessment, Subjective Stress, PTSD, Psychological Distress, Intrusion, Avoidance, Stress Measurement.

Authors

Mardi Horowitz, Nancy Wilner, William Alvarez (Original IES, 1979); Daniel S. Weiss, Charles R. Marmar (IES-Revised, 1997).

Purpose

The primary purpose of the Impact of Event Scale is to provide a reliable and valid quantitative measure of the emotional and psychological impact that a specific, identified stressful or traumatic experience has had on an individual. The scale is designed to quantify the severity of current stress symptoms related to that event, typically focusing on the reactions experienced within the most recent seven-day period.

In clinical practice, the IES is frequently used both as a screening tool to identify individuals who may require professional intervention for trauma-related issues, and as an outcome measure to assess the effectiveness of therapeutic treatments, such as the REMAP process mentioned in the source material. By providing a quantifiable score, clinicians can track changes in a patient’s distress level over time.

Construct

The IES measures the psychological construct of subjective stress reaction following a critical incident. Specifically, the original 15-item IES assesses two core dimensions of post-traumatic stress symptomatology: Intrusion and Avoidance. Intrusion symptoms encompass involuntary thoughts, images, nightmares, and intense emotional waves related to the event. Avoidance symptoms reflect deliberate efforts to suppress thoughts, feelings, or conversations about the event, as well as behavioral avoidance of people, places, or situations that serve as reminders.

The revised version, the IES-R, expands this construct to include a third factor, Hyperarousal (e.g., irritability, difficulty sleeping, hypervigilance), thus aligning the scale more closely with the diagnostic criteria for PTSD outlined in the DSM classifications.

Validity

Both the original IES and the revised IES-R have demonstrated strong psychometric properties, including established validity across diverse populations and traumatic events. The scale has shown high convergent validity, correlating strongly with other established measures of post-traumatic stress and psychological distress. Studies confirm its utility in identifying individuals suffering from significant trauma and less intense forms of stress.

For instance, research has established specific cutoff scores on the IES that strongly correlate with a probable diagnosis of PTSD, confirming its criterion validity as a screening instrument. A score of 27 or higher on the original 15-item IES indicates a 75% probability of having PTSD, while a score of 35 or above is considered the optimal cutoff for a likely diagnosis, underscoring its predictive power.

Reliability

The IES and IES-R are consistently reported as reliable instruments in academic literature. Reliability refers to the consistency of the scale’s measurement, indicating that it yields stable results across repeated uses and measures internal consistency well.

The IES-R, in particular, has been subject to extensive psychometric analysis in various international studies, such as the Japanese-language version (IES-R-J), confirming its reliability and internal consistency across different cultural contexts. The general consensus is that both versions of the scale are robust and dependable tools for measuring trauma-related distress.

Factor Analysis

The original 15-item Impact of Event Scale (IES) is structurally confirmed by factor analysis to comprise two primary subscales:

• Intrusion: Items related to unwanted thoughts, nightmares, and feelings associated with the event (e.g., items 1, 4, 5, 6, 10, 11, 14).

• Avoidance: Items related to deliberate efforts to suppress thoughts or feelings, or behavioral avoidance of reminders (e.g., items 2, 3, 7, 8, 9, 12, 13, 15).

The revised 22-item IES-R introduced by Weiss and Marmar in 1997 is designed to capture three factors: Intrusion, Avoidance, and Hyperarousal. This three-factor model has been consistently supported in subsequent factor analyses, establishing the IES-R as a more comprehensive measure aligning with modern diagnostic criteria for Post-Traumatic Stress Disorder.

Instrument

Test Type: Self-Report Questionnaire / Screening Tool

Format: 15 items (Original IES) or 22 items (IES-R). Items are scored on a 4-point Likert-type frequency scale based on experiences in the past seven days: Not at all (0), Rarely (1), Sometimes (3), Often (5).

Language Available: English, Japanese (IES-R-J), and numerous other translations due to its extensive international use.

Population Group: Clinical and Non-Clinical populations exposed to a specific stressful or traumatic event.

Age Group: Adolescents and Adults.

Population Details: Widely used with survivors of natural disasters, motor vehicle accidents, combat exposure, and various forms of interpersonal violence and psychological trauma.

Test Methodology: The respondent is asked to identify a specific stressful event and then rate how frequently they experienced 15 (or 22) listed symptoms over the past seven days. Scores range from 0 to 75 (Original IES). Interpretation of scores provides an assessment of the severity of the event’s psychological impact.

Keywords

Trauma Screening, Psychological Scale, Horowitz, IES Scoring, Stress Distress, Clinical Assessment, Psychometric Properties, Emotional Impact.

Authors

Author ORCID Identifier: N/A (Information not provided in source.)

Affiliation Email addresses: N/A (Information not provided in source.)

Correspondence Address: N/A (Information not provided in source.)

Permissions & Fee and Test Year

The original Impact of Event Scale (IES) was published in 1979 by Horowitz, Wilner, and Alvarez. The revised scale (IES-R) was published in 1997 by Weiss and Marmar. The IES is generally considered a public domain or open-source instrument for research and clinical use, although specific versions or translations may require permission from the copyright holders. Users should consult the original publication sources for definitive copyright and usage guidelines.

Reference’s

• Horowitz, M. Wilner, N. & Alvarez, W. (1979). Impact of Event Scale: A measure of subjective stress. Psychosomatic Medicine, 41, 209-218.

• Weiss, D.S., & Marmar, C.R. (1997). The Impact of Event Scale-Revised. In J.P. Wilson & T.M. Keane (Eds.), Assessing Psychological Trauma and PTSD (pp.399-411). New York: Guilford.

• Hutchins, E. & Devilly, G.J. (2005). Impact of Events Scale. Victim’s Web Site. http://www.swin.edu.au/victims/resources/assessment/ptsd/ies.html

• Coffey, S.F. & Berglind, G. (2006). Screening for PTSD in motor vehicle accident survivors using PSS-SR and IES. Journal of Traumatic Stress. 19 (1): 119-128.

• Neal, L.A., Walter, B., Rollins, J., et al. (1994). Convergent Validity of Measures of Post-Traumatic Stress Disorder in a Mixed Military and Civilian Population. Journal of Traumatic Stress. 7 (3): 447-455.

• Asukai, N. Kato, H. et al. (2002). Reliability and validity of the Japanese-language version of the Impact of event scale-revised (IES-R-J). Journal of Nervous and Mental Disease. 190 (3): 175-182.

• Creamer, M. Bell, R. & Falilla, S. (2002). Psychometric properties of the Impact of Event Scale-Revised. Behaviour Research and Therapy. 41: 1489-1496.

• Kawamura, N. Yoshiharu, K. & Nozomu, A. (2001) Suppression of Cellular Immunity in Men with a Past History of Post Traumatic Stress Disorder. American Journal of Psychiatry. 158: 484-486

Items of the Impact of an Event scale

The following are the instructions and items for the original 15-question Impact of Event Scale (IES). Respondents are asked to rate how frequently the comments were true for them during the past seven days.

List Today’s Date_________

List the Date of the Event_________

Describe the Event______________________________________________

Below is a list of comments made by people after stressful life events. Please mark each item, indicating how frequently these comments were true for you during the past seven days. If they did not occur during that time, please mark the “not at all” column.