The project “Mechanisms of Panel Conditioning in Longitudinal Studies: Reflection, Satisficing, and Social Desirability” (PaCo) is a joint research project realized in cooperation with GESIS – Leibniz Institute of the Social Sciences, ZPID – Leibniz Institute for Psychology, and Utrecht University. The project addresses the existence, magnitude, and mechanisms of panel conditioning effects – a potential measurement error in longitudinal studies that threatens the validity and quality of panel data. The PaCo-project comprises two data collection streams: a non-probability panel study and a probability-based panel study. In total, the studies comprise 6 consecutive survey waves. This data set and its corresponding codebook comprise the data from the non-probability data collection stream.