Where to look for studies

The next sections run through the sources you are likely to use in finding studies.

Electronic databases

Cochrane collaborative review group specialised registers

Each Cochrane review group is building up a register of studies relevant to its scope. The idea is that instead of each reviewer searching lots of databases and journals for trials relevant to interventions for, say, asthma, it would be more efficient to search these sources centrally for all trials relevant to asthma. The reviewer can then use the register compiled from this work as their first place to search, and they will effectively be searching several sources at once.

Of course, some Cochrane review groups have been around longer than others, and some have more resources to devote to this. So the completeness of these registers varies. You'll need to talk to your review group about getting access to the information in their register.

The Cochrane Central Register of Controlled Trials

This register is part of The Cochrane Library. The idea behind this register is that it should be a central place to put all the reports of controlled trials identified through the work of the Cochrane Collaboration. This means that it contains the results of searching MEDLINE, EMBASE, some other databases and a long list of journals, books and conference proceedings. Many of the reports of studies on the register have been included because they might be reports of trials, based on reading the title and abstract (if there was one).

The content of The Cochrane Central Register of Controlled Trials changes all the time, as does the indexing of entries and retrieval methods. There's more information about this on The Cochrane Library.

Other databases

These are described in section 5.4 of the Reviewers' Handbook, and in the further reading listed at the start of this module.. You'll need some advice from an information specialist about which ones to search.

A particularly important source might be registers of ongoing and unpublished trials.

Handsearching

This means going through journals, books and conference proceedings by hand, looking for relevant studies. Because this takes a long time, the Cochrane Collaboration has encouraged people to register their handsearching and look for all reports of controlled trials. These are then collected and put onto The Cochrane Central Register of Controlled Trials so that no one else has to handsearch that source. The list of journals being searched can be downloaded from the internet at:

www.cochrane.us/documents/master.xls

Again, you may need some guidance from an information specialist about which journals may need searching for your review

Checking references

It's usual to read through the reference lists of any studies you do find, in case the authors have referred to any other relevant studies. It may also be worthwhile looking for previous reviews of the topic and checking their reference lists, too.

Personal communication

People who have been working in a particular topic area may know of studies you haven't yet found. Reviewers commonly send a list of the studies they have found to the authors of those studies, asking if they are aware of any other relevant studies.

Another approach is to write to the manufacturers of relevant drugs or devices and ask if they are aware of any other studies.