Ford, 1987; Schwartz, 1986). Rather, such constructs must be represented by an entire pattern of manifestations (Cattell, 1966). In studying the behavior of living systems, a description and elucidation of the nature of underlying regulatory processes should precede any premature hypothesis testing with group means of individual variables (Barton, 1994). In view of the prevalence and importance of rhythmicity in biological regulatory mechanisms, inclusion of time-varying or temporal aspects of responding is crucial to accurately portray such activity (Glass and Mackey, 1988; Goldbeter and Decroly, 1983; Hrushesky, 1994).

Unfortunately, the analysis of variance (ANOVA) design, a staple of psychophysiological research, confuses temporal information by splitting physiological events into discrete epochs, thereby disrupting the continuity of responding (O’Connor, 1992). Furthermore, precise mapping of physiological activity from these distinct periods onto experimental manipulations is fraught with hazards, such as delayed physiological responses and compensatory homeostatic processes (Levenson, 1988). Alternatively, all recorded activity might be considered as relevant; functional relationships among ongoing physiological processes could then be extracted across observations (O’Connor, 1990).

In response to these various concerns, an alternative framework for research with combat service members is suggested: a multivariate, systems perspective that emphasizes the study of individuals. A distinctive feature of this approach is its focus on intraindividual variability in the behavioral and physiological processes of an organism. Moreover, within-group variance (typically treated as error in traditional experimental psychology) is also investigated since it contains a wealth of relevant information (Cronbach, 1957). Thus it is appropriate first to address these basic methodological issues in design and analysis for research involving combat service members.

SEX AND GENDER

Sex and gender factors, including genetic, hormonal, and behavioral factors, are important variables that contribute to differences in biological responses in all species. Sex and gender should therefore always be considered and taken into account when designing and analyzing studies at all levels of biomedical- and health-related research (IOM, 2001b). Data related to sex and gender variables in health and disease have been recently summarized in an Institute of Medicine report (IOM, 2001b). This report pointed out that incidence and severity of diseases vary between the sexes and may be related to differences in genetic, hormonal, cellular, or behavioral responses, as well as to differences in exposures, routes of entry, or the processing of a foreign agent. The report also underscored the importance of performing studies at different stages of the life span to determine how sex and developmental differences influence health, illness, and longevity.

Several examples of sexually dimorphic biomarkers are relevant to this report. For example, in the context of a military setting, it is known that the inci-



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