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. The trial doesn’t involve a specific intervention other than normal standard care.

Examples of an observational study include looking at:

  • How many people have lipodystrophy at one time, or
  • How many people develop lipodystrophy over time

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

Examples of an experimental study include:

  • Comparing whether switching one drug for another improves diarrhoea or another side effect
  • Seeing whether diet or exercise can improve fat accumulation

Cross-sectional vs longitudinal

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

Examples of a cross-sectional study include:

  • Looking at a group of patients to see how many people have osteoporosis (bone disease), or
  • Finding out what percentage of HIV-positive patients are smokers

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

Examples of a longitudinal study include:

  • Following a group of patients to see how many develop lipodystrophy
  • Following a group of patients to see whether an intervention to quit smoking could reduce the percentage of patients at risk of heart disease

Retrospective vs prospective

A retrospective study looks backwards in time.

Examples of a retrospective study include:

  • Analysing a database to find out what percentage of patients failed their first combination, or
  • Looking at medical records to see whether a recently reported side effect occurred in other patients

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

Examples of a prospective study include:

  • Comparing a new HIV drug to an existing drug, or
  • Following a group of patients to see whether heart disease is linked to HIV treatment

In 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: 21 July 2009.