Now that I’ve written about why randomized controlled experiments are so great, it’s time to talk about some of the common ways in which they can go wrong. But first I’d like to make an important caveat: finding potential flaws with any research, even randomized controlled experiments, is actually pretty easy. I haven’t come across any study that couldn’t be criticized on one or more grounds. So with the power to criticize also comes great responsibility: don’t use it to dismiss results you don’t like. Don’t selectively apply these criticisms to some studies and accept the findings of others that could be subject to similar criticisms. Use the knowledge wisely.

The main concern with randomized controlled experiments is the question of “external validity”. Sure, you’ve shown that something works in the laboratory or in a carefully controlled setting, but does it work in the real world? If people in the laboratory are different from those who will be subject to the treatment in the real world or if people (including those administering the treatment) behave differently in the experiment.

For example, maybe you run a clinical trial for a drug and only recruit men to participate in the trial. Will the drug work as well on women? Will there be different side effects for them? For a long time, clinical trials frequently omitted or under-enrolled women, although that is now changing. Or maybe you enroll obese individuals in a weight-loss trial but only includes ones without other health problems like diabetes. But once the drug goes to market, it may be prescribed to all types of obese individuals, and potentially have different effects than what you observed in the laboratory. Or maybe the nurses working on your trial are really good at getting patients to take the drug on time, but in the real world people forget to take it and you observe much lower effectiveness.

External validity is a potential problem with all experiments, not just clinical trials and not just stylized laboratory experiments. As long as people know they are part of an experiment, they may change how they act (maybe to make the experimenter happy, maybe to hide socially unacceptable views or behaviors, or maybe because they don’t take the experimental treatment as seriously as they do things in the real world). This is known as the Hawthorne effect, and it’s essentially impossible to rule out unless your subjects do not know that they are being studied.

Finally, external validity can also be a concern if you’re trying to say something about high-stakes decisions by running a low-stakes experiment. For example, you’re open to this criticism if you want to say something about how people save for retirement and you either run a hypothetical choice experiment or an experiment with low stakes (because who can afford to run an experiment where tens of thousands of dollars are at stake?). In some cases, the low-stakes findings survive in a high-stakes environment, but in others they don’t.

The bottom lines is that the most convincing experimental conclusions are those that are based on a representative population that faces stakes similar to what they would be in the real world, and where the experiment closely resembles real-world conditions (including individuals being unaware that they are part of an experiment).