The Impact of Events Scale

The featured paper is from 1979 and is on the Impact of Events Scale. This is a scale which is used to assess the impact that a trauma has had on a person and can be used in the work-up of people with Post-Traumatic Stress Disorder (as well as related conditions) who undergo treatment. The paper is by Mardi Horowitz and colleagues.

The authors begin by identifying questionnaires that look at the impact of life events on individuals and quote the classic Holmes and Rahe social readjustment rating scale of 1967 (Holmes T, Rahe R, 1967). They then identify the need to have a rating scale that relates to the person’s subjective experience of the trauma and outline the various merits (which have been borne out in practice over the subsequent 3 decades).

The authors describe how the responses of intrusion and avoidance (this term is used instead of denial) were identified as themes from detailed interviews. The aims of the study are clearly stated, being to test reliability of the scale, check the frequency of responses and to test the intrusive and avoidance subscales empirically.

66 subjects (50 women, 16 men) were chosen with an average age of 34 years from referrals for ‘stress management syndromes’ to a clinic in San Francisco and were of ‘lower-middle to middle class..with diverse ethnic origins’.

There was a frequent response to most items on the scale ranging from 85% on ‘I got waves or pangs of intense or deep feelings about it’ to 36% on the lowest rated item. Women in the study endorsed three of the items more than the men including an item about keeping feelings ‘under wraps’. A cluster analysis supported the intrusion and avoidance subscales whilst the items in both subscales were reduced by retaining only those whose correlation with the subscale total score was less than 0.01. The correlation between subscales on the revised test was 0.42 which the authors interpreted as being low enough to ensure they were measuring  different dimensions but high enough for there to be an association between the constructs.

The theme of dissecting cadavers came up when analysing the scale. Thus some physical therapy students who had begun dissection of a cadaver as part of their studies were administered the test twice over a week and referred to the prior experience of dissecting the cadaver which the authors anticipated would be a stressful event. There was a good test-retest reliability with the intrusion subscale (0.89) being more reliable than the avoidance subscale (0.79). Sensitivity was assessed by looking at the effects of psychotherapy on the patient group which did indeed show an anticipated reduction of scores after the intervention.  Medical students who had begun their cadaver dissections were compared against the patients and had much lower scores again providing evidence that the scale was detecting what it should have been (although this seems to assume that medical students shouldn’t find dissecting a cadaver distressing presumably as there is self-selection in this regards – this also raises the possibility that people have an innate ability to identify which stressors they can manage and make long-term career choices accordingly).

This in my opinion is a classic paper as it relates to a rating scale that is widely used because of its clinical utility. There are many useful insights into the development of the scale particularly the assessment of sensitivity and test-retest reliability.

References

Holmes T, Rahe. The social readjustment rating scale. Psychosomatic Research. 11. 213-218. 1967.

Mardi Horowitz, Nancy Wilner and William Alvarez. Psychosomatic Medicine. Vol 41. No 3. May 1979. 209-218.

Disclaimer

The comments made here represent the opinions of the author and do not represent the profession or any body/organisation. The comments made here are not meant as a source of medical advice and those seeking medical advice are advised to consult with their own doctor. The author is not responsible for the contents of any external sites that are linked to in this blog.

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