RCTs are not the only way to obtain evidence of causality.

In causal inference, limited causal claims can be made through observational studies (where no control is done), such as retrospective natural experiments [1]. These methods are of course limited by assumptions and by how closely they match reality.

Note: while a controlled experiment is the gold standard, it is not always feasible or necessary. We don't need to do RCTs on parachutes to prove their efficacy for instance. [2]

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[1] https://en.wikipedia.org/wiki/Natural_experiment

[2] https://www.scientificamerican.com/article/volunteers-jumped...

When I took an epidemiology course in college, our professors taught us about both control studies and cohort studies. Cohort studies might be an example of what you're looking for. This article [0] hosted on the NIH's site discusses cohort studies and their effectiveness / pitfalls.

References: [0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2998589/