David Watson, Lee A. Clark, and Auke Tellegen, «Development and Validation of Brief Measures of Positive and Negative Affect: The PA — NAS Scales», Journal of Personality and Social Psychology 54, no. 6 (1988): 1063–1070; David Watson and Auke Tellegen, «Toward a Consensual Structure of Mood», Psychological Bulletin 98, no. 2 (1985): 219–235.