In a 2018 study reported in Child Development, Ballard et al. examined links between civic engagement (voting, volunteering, and activism) during late adolescence and early adulthood, and socioeconomic status and mental and physical health in adulthood. The researchers studied how civic engagement was associated with outcomes among 9471 adolescents and young adults. They found that all forms of civic engagement are positively associated with subsequent income and education level. Was this a controlled experiment or an observational study? Explain.