1 Department of Environmental and Community Medicine, University of Medicine and Dentistry of New JerseyRobert Wood Johnson Medical School, Piscataway, NJ 08854.
2 Department of Human Ecology, Rutgers University, New Brunswick, NJ 08903.
3 Department of Psychology, Graduate School, Loma Linda University, Loma Linda, CA 92350.
4 Division of Epidemiology, School of Public Health, University of Medicine and Dentistry of New Jersey, Piscataway, NJ 08854.
In a recent commentary (1), Dr. Lea Steele provided a valuable survey of evaluations of the health of Persian Gulf War veterans. Investigating whether veterans of the Gulf War exhibit adverse health effects from their Gulf service, she questioned the use of exploratory factor analysis (EFA) (27) and recommended an alternative approach for investigation. Specifically, Steele stated that 1) "epidemiologic methods" (1, p. 408) are preferable to EFA in determining the presence or absence of a unique syndrome and 2) comparisons among veterans deployed to the Gulf theater and veterans deployed elsewhere should be the most informative. She defined epidemiologic methods as "generating a description of the illness and then delineating its distribution in a given population according to characteristics of person, place, and time" (1, p. 408), as in her study of Kansas-based Gulf War-era veterans (8). In that study, she defined symptom groups as reliable if they had a Cronbachs of 0.7 or greater and an item-scale correlation of at least 0.5 (8, p. 993). Steele then defined illness as having "moderate or multiple symptoms in at least three of the six defined [symptom] groups" (8, p. 995) and found a prevalence of approximately 34 percent. She preferred this "epidemiologic" approach because, in her use of EFA, "The cooccurrence of symptoms in different categories... varied in veteran subgroups" (8, p. 993).
We do not understand Steeles preference for her "epidemiologic" method over EFA, and she does not justify it empirically. First, EFA is an objective, empirical approach, while Steeles "epidemiologic" method depends on the investigators specification of appropriate categories of symptoms, according to unspecified criteria. Second, because Steeles symptom groups are apparently mutually exclusive, her approach will not identify the co-occurrence of symptoms from different symptom groups. For example, hypothetically, nausea symptoms could result from an underlying gastrointestinal disorder in some subjects and could be one of a constellation of responses to a chemical agent in others. In the first case, it might be grouped with other gastrointestinal symptoms, while in the latter it might be grouped with neurologic or musculoskeletal symptoms. Steeles method would assign it to only one group, while in EFA the symptom could contribute to more than one factor. Third, Steele rejected EFA because results differed among groups of veterans by deployment status and gender. When such heterogeneity is identified, it is prudent to separately analyze homogeneous groups rather than pool the data or change the methodology.
To assess the robustness of EFA, we applied six different EFA implementations to two data sets (3, 7) and found that results differed only in small ways (9) and were similar to Steeles. All methods provided useful and similar descriptions in this instance, which is not surprising given the relative homogeneity of the subjects (Gulf War veterans).
We defined illness by conducting a two-group k-means cluster analysis on our four factors and found a prevalence similar to Steelesapproximately 38 percent (7). We obtained similar results when we applied the same method directly to all 48 symptoms (table 1), with nearly 95 percent of subjects identically classified and a kappa value of 0.9. This suggests that EFA, while useful descriptively, was not necessary to define illness.
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