Clinical Trials

Blinded trials

There are several ways in which the results of trials can be made as reliable and accurate as possible. One of these is to make the trial a ‘blind trial’. In a blind trial the participants are not told which group they are in. This is because if they knew which treatment they were getting, it might influence how they felt or reported their symptoms. Some trials are double-blind, which means that neither participants nor the doctors treating them know which people are getting which treatments. This also avoids the doctors’ hopes and expectations influencing the results of the trial.
Amanda explained how the yoghurt cartons were labelled with a number so no-one could tell whether they contained the pro-biotic yoghurt or a placebo version. Phil was in a study which combined a comparison of named drugs for high blood pressure and a blinded comparison of a cholesterol-lowering drug and a placebo.
Kate and her husband also began to guess which arms of the trial they were in, comparing grass pollen injections with placebo to build up resistance to hay fever.
Kate and her husband were proved right about which arm of the trial they were in. However, it is possible that someone who is taking a placebo may notice some improvements in their health. This is known as the placebo effect or placebo response. It is also possible that someone taking the active ingredient may notice no change.
As Kate explained, her husband was offered a course of the active injections after the trial ended, because it had shown they did improve hay fever symptoms. But she also noted that one problem with guessing which treatment you are on is that it may make some people want to stop taking part. Her husband did in fact continue, but it was a big commitment, knowing he was on the placebo. (See also ‘Feeling about being in a placebo-controlled trial’ and ‘Thinking about withdrawing from a trial’).
Normally blinding requires that the active treatment and the placebo should look as similar as possible. Amanda tried to set up an ‘n of 1’ or ‘number of 1’ trial on treating her rheumatoid arthritis. She would have been the only participant and would have switched between a placebo and a drug she had not taken before, without knowing which she was taking. (This is known as a crossover trial design). In the end the trial did not go ahead, but she was struck by the fact that the placebo did not look like the active drug.
She said “It’s made me much more sceptical about trials… I’m also sceptical about trial results because people want things to happen.” On the other hand Danny noted a rather different problem. She and her husband both started the same trial for high blood pressure medication. In the end he was ineligible and she dropped out later because of side effects. But she wondered what would have happened if they had ever got their tablets mixed up. “We were both given a packet and there was no way of differentiating his packet and mine.”
Sometimes it is not possible to conceal what treatment someone is getting if there are very obvious differences. For example, the trial Jenny was in was comparing different medications for heavy periods with fitting a coil or intrauterine device (IUD). In Judith’s trial, patients were randomised to have chemotherapy every two or every three weeks. Even if treatments cannot be blinded from the doctors or patients, one possible solution is to send the results away for analysis, so the person analysing them does not know the patients or what treatment they have had.
A lot of the terms used in trials are quite complex and closely related to each other. It was evident in our interviews that people sometimes got them mixed up, even including some well-informed people who had worked in health care. For example, people asked about their understanding of why randomisation was used might give a good explanation of why a control group was needed. Sometimes people were confused about the difference between placebos and controls, or between randomisation and blinding. Below we give some brief definitions as a reminder of how they fit together (See also ‘What are clinical trials and why do we need them?’ and Resources).
Control group – a comparison group in which people often get a standard treatment. If no standard treatment exists the control group receives no specific treatment or a placebo. Comparing the results of people in the control group with people in the experimental group helps assess whether there are differences between the new treatment and the control treatment. (See also ‘Feelings about being allocated (randomised) to a treatment group').

Randomisation – allocating people at random to one group or another, so that each group contains a similar mix of people. Random allocation helps ensure we are comparing two very similar groups of patients, so if one group does better than another, it is very likely to be because the treatments being compared have different effects, and not because of differences between the people in the groups. (See also ‘Feelings about being allocated (randomised) to a treatment group').

Placebo – a treatment with no active ingredient which is designed to appear very similar to the treatment being tested. By comparing people’s responses to the placebo and to the treatment being tested, researchers can tell whether the treatment provides any additional benefit. (See also ‘Feelings about being in a placebo-controlled trial’).

Double-blinding – making sure neither patients nor doctors know which treatment each person is getting, so that this knowledge does not influence how patients feel or how doctors interact with their patients or interpret the results.

Last reviewed September 2018.
Last updated July 2011


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