Why are randomized controlled trials the gold standard?

A randomized controlled trial (RCT) is called the gold standard because random assignment balances the characteristics of participants across groups, including ones nobody measured. That lets researchers attribute differences in outcomes to the intervention rather than to other factors. According to [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC6235704/), this is not possible with any other study design, although no single study is likely to prove causality on its own. [1]

In an RCT, participants are assigned by chance, in a process the [iwh.on.ca](https://www.iwh.on.ca/what-researchers-mean-by/randomized-controlled-trial) explainer compares to flipping a coin, to an intervention group or a control group. The control group may receive a placebo, conventional treatment or nothing. Because allocation is random, it minimizes the chance that the two groups differ in traits such as age or disease, so outcome differences can be attributed to the intervention. [2]

The key contrast is with observational studies. In non-randomized research it may be possible to adjust for known confounders, but unknown or poorly measured ones are much harder to handle, according to [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC8176647/). A further source, [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC1706053/), notes that statistical techniques may not control confounding adequately when the confounding factor is measured badly. [3] [4]

Randomization works best alongside other features. Concealment means nobody knows at recruitment which group a participant will join, often ensured with computer-generated allocation. Blinding keeps participants and staff from knowing who receives which treatment. Analysis by intention-to-treat, in which people are analyzed in the groups they were randomized to, is often regarded as least biased. Trials should also have pre-specified primary outcomes, be registered, and have ethical approval, as described by [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC6235704/). [1]

Poor execution undermines the status. A commentary at [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC4840795/) says an RCT can produce biased results without methodological rigor, and that inaccurate methods were used in many trials for years, which led to reporting guidelines. [5]

RCTs are expensive and slow. Volunteers may not represent the wider patient population, and strict inclusion criteria can produce a homogeneous group. Trials also tend to test treatments in idealized settings that differ from real-world use, and participants can be lost to follow-up, according to [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC8176647/) and [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC6235704/). [3] [1]

Some questions cannot ethically be tested this way. The [science.health.ny.gov](https://science.health.ny.gov/faq/3) page gives the example of a suspected harmful food additive: assigning people to consume it would be unethical, so other designs have a role. That source places experimental studies at the top of the evidence hierarchy and treats other designs as able to show association only. [6]