Relationship between variables: correlation/association and causation

Hypothetical questions involve variables (or assigned variables). Hypotheses test the relation of these variables. There are two main types of relation: correlation/association and causation.
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Causation is more difficult to test as compared to correlation/association. In clinical trials, controlled experiments are designed to test causation whereby sampling of a population produces treated and untreated groups and the hypothetical question is whether a treatment will cause specified response. The variables are therefore (1) treatment and (2) response, whereby the sample is assumed to be similar.
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Several good reviews on the subject are the following:
1. https://www.psychologytoday.com/blog/all-about-addiction/201003/correlation-causation-and-association-what-does-it-all-mean