The concept of random assignment of the exposure or treatment as the mechanism that controls for unwanted sources of variation between exposure groups/levels and allows causal inference Why with ...

They also looked at scoring-row placement – grouped versus separate – and variations within-participants and scores – present versus absent – and between-participants by random assignment. The study ...

Why with observation alone there is no random assignment of exposure, so confounding by other factors should always be considered as an explanation for associations found in an observational study.