The average causal effect is a parameter that can only be estimated from which type of study?

The average causal effect is a parameter that can only be estimated from which type of study?


Options:


Observational study with random sampling


Poll


Observational study without random sampling


Randomized experiment|||You are here in Botany, therefore I`m not sure, but suppose Observational study without random sampling is correct.


Please read


Introduction of this article


https://docs.google.com/viewer?a=v%26amp;q=cac鈥?/a>





And/or


"Average Causal Effects From Nonrandomized Studies" %26gt; https://docs.google.com/viewer?a=v%26amp;q=cac鈥?/a>