#scope-of-inference

Question 1: Single choice

Researchers recruited 600 asthma patients aged 18–69 who relied on medication (they were not randomly sampled) and randomly assigned them either to practise a breathing technique or not. The technique group showed a significantly larger reduction in symptoms. Which conclusion is justified?

It shows a causal effect, but extending it to all such patients assumes the recruits represent them.1
No causal conclusion is possible, because the patients were not a random sample.2
It shows a causal effect that holds for all asthma patients aged 18–69, with no further assumptions needed.3
It shows only an association, because this is an observational study.4

Timed test