Treatment training manual

8. 6 Main types of trial design

There are three main ways to categorise research.

Each type of study has specific advantages and disadvantages.

They each provide different types of information.

Observational vs experimental (or interventional)

An observational study either looks for evidence that something has happened, or follows people to see whether something happens in the future. The trial doesn’t involve a specific intervention other than normal standard care.

Examples of observational studies include:

  • Finding out how many people have a certain side effect at one time.
  • Finding out how many people develop a certain side effect over time.

An experimental (or interventional) study is where something specific is done in the study. For example, using a treatment, strategy, or other intervention, that is recorded and analysed.

Examples of experimental studies include:

  • Comparing whether switching one drug for another improves a side effect.
  • Seeing whether diet or exercise can lead to weight loss.

Cross-sectional vs longitudinal

A cross-sectional study collects information at one point in time.

Examples of cross-sectional studies include:

  • Looking at a group of people to see how many have osteoporosis (bone disease).
  • Finding out what percentage of people living with HIV are smokers.

A longitudinal study follows individuals to see how things change over time.

Examples of longitudinal studies include:

  • Following a group of people to see how many develop diabetes.
  • Following a group of people to see whether an intervention to quit smoking reduces the risk of heart disease.

Retrospective vs prospective

A retrospective study looks backwards in time.

Examples of retrospective studies include:

  • Analysing a database to find out what percentage of people had viral load failure on their first combination.
  • Looking at medical records to see whether a recently reported side effect had previously occurred.

A prospective study decides on what is going to be studied and then follows people over time to see what happens.

Examples of prospective studies include:

  • Comparing a new HIV drug to an existing drug.
  • Following a group of people to see whether heart disease is linked to HIV treatment.

When describing a study, one of each of these three terms should be included.

For example:

  • An observational, longitudinal, prospective study.
  • An interventional, longitudinal, prospective study.
  • An observational, cross-sectional, retrospective study.


Last updated: 1 January 2023.