In a study, the participant is blinded if they do not know which treatment they are on (Drug A or Drug B). The researcher and the doctor can both be blinded as well, meaning they do not know which treatment the participant is on.

The reason for blinding is to make sure that there is no bias in measuring the results. For example, if a participant knows they are on a new drug, and if pain is being measured, it might affect how they report the amount of pain they are feeling.

Safety precautions are built into a trial to allow everyone to find out which treatment a participant is on if required.

Clinical Trial

A particular type of research study that looks at the difference between (at least) two treatments.

The main treatment might be:

  • A new drug that is being developed
  • A drug that is already used but is being tested for a different condition/disease or at a different dose
  • A new programme that has been developed to help address some patient problem. Such a programme might be a weight management programme, or a smoking cessation programme or a support programme to help patients with diabetes to self-manage their condition better.

In a clinical trial, the main treatment is compared either to another treatment (so e.g. Drug A v Drug B) or to what is called usual care. Usual care means the patient continues to get whatever treatment or care he/she normally receives – so this might be a drug or it might be no drug, or it might be no support programme.

Randomisation is a very important part of clinical trial – check out what Randomisation means.


The Health Products Regulatory Authority, the organisation in Ireland who gives approval to allow a clinical trial involving a drug to go ahead.

Principal investigator (PI)

The overall leader of a research team, usually an academic who is a very experienced researcher, working in a particular disease area for a number of years.


In a clinical trial, those taking part are randomised at the start of the trial to decide which treatment they get. Think of it like flicking a coin: heads you get one treatment, tails you get the other treatment. Slightly more sophisticated methods are used to randomise participants in trials, but all are based on the same principal as flicking a coin.

The  reason for randomisation is so that the participants on treatment 1 should be, on average, more or less the same at the start (in terms of age, gender, how sick they are, education and income levels etc.) as those on treatment 2.