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Clinical trials and systematic reviews


These websites provide information about or enable you to search for clinical trials:

Pharmaceutical company registers:


If you have been searching for references in other databases like Medline, you may be able to limit your results to only those that are systematic reviews or clinical trials. See the section on using search filters.


The Centre for Reviews and Dissemination (CRD) offers the following useful definitions*:

Controlled trial
A clinical trial with a control group.

Control group
The group that acts as a comparator for one or more experimental interventions in a controlled trial. The group without the disease or outcome of interest in a case control study.

Randomised controlled trial
An experiment in which investigators use randomisation to allocate participants into the groups that are being compared. Usually allocation is made at individual level, but sometimes it is done at group level, eg by schools or clinics.

Systematic review
Systematic reviews aim to identify, evaluate and summarise the findings of all relevant individual studies, making the available evidence more accessible to decision-makers. When appropriate, combining the results of several studies gives a more reliable and precise estimate of an intervention's effectiveness than one study alone.

Systematic reviews adhere to a strict scientific design based on explicit, pre-specified and reproducible methods. Because of this, when carried out well, they provide reliable estimates about the effects of interventions so that conclusions are defensible. As well as setting out what we know about a particular intervention, systematic reviews can also demonstrate where knowledge is lacking. This can then be used to guide future research.

*CRD. 2008. Systematic reviews : CRD's guidance for undertaking reviews in health care. York: CRD, University of York