Between- and Within-person Variation in Affect and Personality over Days and Years: How Basic and Applied Approaches Can Inform One Another*

Distinguishing within-person from between-person variability in personality and wellbeing constructs is important for understanding their stability and change over varying periods, from moments to decades. Regardless of whether the variation is over days or years (or seconds to decades), it is important for researchers and practitioners alike to comprehend what it means for a person to vary from himself or herself, and what it means for one person to vary from another. The current study gives two examples of withinand between-person. The first example involves two major personality traits, extraversion and neuroticism, over a 12-year longitudinal period. The second example involves negative affect in a one-week daily diary study. Researchers should be aware of the within vs. between distinction because it is at the heart of important theoretical problems in ageing and development. Practitioners should appreciate it because it can distinguish between long-term clinical trends and short-term fluctuations.