Emotional variability is considered by many personologists a theoretically important characteristic of persons. Variability is conceptualized as frequent and extreme changes in mood or emotion over time. Many researchers index mood variability by a within-subjects standard deviation computed from mood ratings provided by single subjects on several occasions. In this article, 1 argue that this index is theoretically limited because it refers mainly to the average extremity of mood change and not the frequency of change. Because the frequency of change is important to any theory concerning emotional variability, I suggest that time-series techniques be used to better represent this aspect of the construct. The studies here illustrate how one class of time-series techniques-spectral analysis-can be applied to daily mood measures to index the frequency of mood shifts. The convergent validity of spectral estimates of mood change frequency is assessed relative TO self-report and parental-report measures of emotional reactivity. The divergent validity of the spectral estimates of mood change frequency is assessed by examining their relation to the within-subjccts standard deviation computed from daily mood assessments. Results suggest that the spectral approach provides an adequate index of mood change frequency that is independent of mood change extremity, helps clarify the meaning of emotional variability, and provides a new class of analytic strategies for inclusion in the idiographic-nomothetic paradigm.Variability in behavior and emotion has attracted the attention of personologists working within widely different traditions. These investigators have all assumed that variability is an inherent and stable characteristic of persons. For example, Henry Murray (1938Murray ( /1981 wrote, "In studying a person over[time], it is assumed, as an approximation, that his personality remains potentially the same. The sometimes marked inconsistencies that occur are put down to the subject's characteristic range of variability, itself an attribute of personality" (p. 110, emphasis added). Fiske (1961) observed, "Holding constant the general conditions and the specific stimuli, we can observe dependable individual differences in variability of response" (p. 340, emphasis added). In a discussion of emotion, Cattell (1973) suggested that "A general tendency to fluctuate . . .exists as a personality trait" (p. 225).Although many investigators are concerned with processes of change and variability, agreement as to what are satisfactory procedures for denning and studying variability is far from widespread (Zevon & Rounds, 1985). Thus, even though many would agree that variability, particularly emotional variability, I wish to thank Jerry Busemeyer, Ed Diener, Alexander Von Eye, and two anonymous reviewers for their helpful comments on an earlier version of this article. Conversations with Robert Bohrer and John Grottman also contributed to ideas developed in this article.