8 Clinical trials and research

Main types of trial design

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There are 3 main categories that cover all trials. Each type of study has specific advantages and disadvantages. They all 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 a group of patients to see how many people have lipodystrophy
• Following a group of patients to see how many people develop lipodystrophy

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
• Seeing whether diet or exercise can improve fat accumulation

Cross-sectional vs longitudinal

A cross-sectional study looks at collecting 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) at one time point
• 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
• Looking back at medical records to see whether a recently reported side effect has happened 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
• A trial of New-fill to treat facial fat loss (lipoatrophy)
• 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, ie:

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