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Masking or blinding is a concealment technique that prevents the existence of information biases.
It’s said that there’s none so blind than those that refuse to see. But it’s also true that wanting to see too much can be counterproductive. Sometimes it is better to see just the essential and indispensable.
That’s what happens with scientific studies. Imagine that we want to test a new treatment and we propose a trial to some people, giving the new drug to some of them and a placebo to the rest. If we all know what is treated each with, it might be that researchers or participants expectations influence, even inadvertently, the way we evaluate the results of the study. This is why you have to use masking techniques, better known as blinding.
Let’s suppose we want to test a new drug for treating a very severe disease. If a participant knows he’s receiving the drug he will be much more tolerant with side effects than if he receives placebo. And something similar can happen to the researcher. It’s easy to imagine that you would take less interest in asking for a toxicity sign to a patient that you know is being treated with a harmless placebo.
All of these facts may influence the way participants and researchers evaluated the effects of treatment and may lead to a bias in interpreting results.
Masking techniques can be performed at different levels. The lowest level is not masking at all, making what is called and open or un-blinded trial. Although masking is the ideal thing to do, there’re times when it’s not possible or convenient. For example, think about the case you need to cause unnecessary inconvenience to the patient, such as administering an intravenous placebo for a long time or doing a sham surgical procedure. Other times it’s difficult to find a placebo galenicaly indistinguishable from the drug tested. And finally, sometimes it doesn’t make much sense to blind if treatment produces easily recognizable effects that don’t occur with placebo.
Types of blinding
The next level is the single-blind, when either participants or researchers don’t know which treatment is receiving each one of them. A further step is the double-blind, in which neither researchers nor participants know which group each one is assigned to. And finally, we can do a triple-blinding when, in addition to the aforementioned, the person who analyze the data or who has the responsibility to control and stop the study also unknowns which group each participant is assigned to. Imagine someone has a serious adverse effect and we have to decide if we must stop the study. No doubt that knowing if that person is receiving the drug or placebo can influence our decision.
But what can we do when masking is not possible or is inconvenient?. For such cases we have no more choice than to make an open or un-blinded study, although we can try to use a blind evaluator. This means that, although researchers and participants know the allocations to placebo or intervention groups, the person who analyzes the results doesn’t know it. This is especially important when the outcome variable is a subjective one. By the way, it’s not so essential when we measure objective variables, such as a laboratory determination. Think that you won’t assess an X-ray film with the same detail or criteria if you know that the individual comes from the placebo or the intervention group.
To end this post, we are going to discuss two other possible errors resulting from lack of blinding. If a participant knows he’s receiving the studied drug he can improve just by a placebo effect. On the other hand, if he knows he’s in the placebo arm, he can modify his behavior just because he knows “he’s not protected” by the new treatment. This is called contamination and it’s a real problem in studies about lifestyle habits.
And that’s all. Just to clarify a concept before the end. We have seen that there is some relationship between lack of blinding and the appearance of a placebo effect. But don’t be mistaken, masking is not the way to control the placebo effect. For that we have to resort to another trick: randomization. But that’s another story…