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“Ok, I agree that ‘correlation doesn’t prove causation’ in general, but in a case like this where we have lots of other kinds of evidence it sure gives us a pretty strong guess about causation.”

“There is an answer to this causal question.  Just because we can’t ethically do a randomized controlled study with these patients, it doesn’t mean that we can’t make progress establishing the causal link between these treatment options and the outcome.   After all, we have pretty good evidence that the energy from the sun is caused by nuclear fusion and we haven’t done any randomized controlled experiments!”

"One hundred years after that, French chemist Antoine Lavoisier used a device called an “ice calorimeter” to gauge the energy burn from animals —like guinea pigs — in cages by watching how quickly ice or snow around the cages melted. This research suggested that the heat and gases respired by animals, including humans, related to the energy they burn."

Students tend to think that one flaw in a study makes the study useless as evidence, rather than merely weaker evidence than it would be without the flaw.

Students struggle to generate non-RCT types of evidence for causality, although they are better at recognizing it.

Identifying natural experiments is also difficult, probably because many students find the principles of RCTs slippery.

"Since we can't run a randomized controlled trial on whether CO2 emissions cause global warming, we can't ever know whether it does."  

There are non-RCT forms of evidence for causal hypotheses, which are less conclusive than RCTs but together can offer strong evidence for causation. These include:

Identify additional sources of evidence that could be used to help mitigate flawed experiments, including prior plausibility, dose-response relationships, size of effect, temporal ordering, and multiple complementarily-flawed experiments.