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while most researchers do agree now that situations may have an effect in the assessment of traits, the consequences have been neglected, so far: if situations aflect the assessment of traits we have to take this fact into account in studies on reliability and validity of measurement instruments and their application. In the theoretical part of this article we provide a more formal exposition of this point, introducing the basic concepts of latent state-trait (LST) theory. LST theory and the associated models allow for the estimation of the situational impact on trait measures in non-experimental, correlational studies. In the empirical part, LST.theory is applied to three well known trait questionnaires: the Freiburg Personality Inventory, the NEO Five-Factor Inventory and the Eysenck Personality Inventory. It is shown that significant proportions of the variances of the scales of these questionnaires are due to situational effects. The following consequences of this finding are discussed (i) Instead of the reliability coefficient, the proportion of variance due to the latent trait, the consistency coefjcient, should be used for the estimation of confi&nce intervals for trait scores. (ii) To reduce the situational effects on trait estimates it may be useful to base such an estimate on several occasions, ie., to aggregate data across occasions. (iii) Reliability and validity studies should not only be based on a sample of persons representative of those to whom the test will be applied; they should also be conducted in situational contexts representative of the intended applications.

The advantages of assessing self‐perceived deviations of mood states from mood traits in cross‐sectional as well as longitudinal studies are discussed in detail. The validity of self‐perceived mood deviations is analysed in a longitudinal study with three occasions of measurement and 176 participants. In order to assess self‐perceived mood deviations, participants rated the deviation of their momentary mood from how they felt in general with respect to 32 mood adjectives belonging to three mood scales (pleasant–unpleasant, calm–restless, awake–sleepy). Furthermore, mood states, mood traits, and other personality variables (hedonic level, neuroticism, extraversion, openness, agreeableness, conscientiousness) are assessed by self‐report. Using models of latent state–trait theory, it is shown that the self‐perceived mood deviation scales are reliable (reliabilities between 0.73 and 0.95) and sensitive to occasion‐specific fluctuations (occasion specificities between 0.38 and 0.72). The self‐perceived deviation scales show high correlations with latent occasion‐specific deviation variables defined on the basis of repeatedly measured mood states (high convergent validity). In contrast to self‐reported mood states, however, self‐perceived mood deviations show much smaller and in most cases non‐significant correlations with personality variables (high discriminant validity). Furthermore, it is shown that self‐perceived mood deviations can be used to suppress stable situation‐specific variance in mood traits defined as aggregated states. Therefore, it can be demonstrated that including self‐perceived mood deviations in analyses on mood–personality associations enhances the association coefficients considerably. Finally, the implications of the results for the individual assessment of mood deviations, as well as for studies on affect and personality, are discussed. Copyright © 1999 John Wiley & Sons, Ltd.