b. Randomized experiments is considered as the gold standard for determining causal inference.
More about Randomized experiments:
In a randomized experiment, a research sample is split into two groups: one that will get the intervention under study (the treatment group), and the other that won't (the control group). All of the city's registered voters, for instance, might make up the study sample.
Randomly assigning study subjects, such as individual voters, precincts, media markets, or other groups, to treatment or control groups is the main component of randomized experimental research designs.
In this situation, randomization has a very special meaning. It doesn't mean arbitrarily or carelessly picking some people and ignoring others.
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