Gable, S. L., & Nezlek, J. B. (1998). Level and instability of day-to-day psychological well-being and risk for depression. Journal of Personality and Social Psychology, 74, 129-138.

For 21 days, 123 undergraduates provided measures of their self-esteem, anxiety, causal uncertainty, perceived control over outcomes, and three constructs comprising Beck's cognitive triad. Factor analyses of the mean level and day-to-day instability of these measures produced two factors, level and instability of well-being. Participants also provided four measures of their risk for depression. For participants who were classified as not at-risk for depression, level of day-to-day well-being was negatively related to level of depressive symptoms, and this effect was not moderated by day-to-day instability. In contrast, for participants who were at-risk for depression, the negative relationship between well-being and depressive symptoms was moderated by instability; it was stronger for those who were more unstable.