The first step in any PhD is to find, read and critique the literature, in other words, a literature review.  The body of work reviewed might include published scientific papers, books, government or institution reports (called grey literature), or other theses. Consider using bibliographic software such as Endnote for storing information about each reference. There are in fact several type of literature review methods available.

Cochrane review

According to the Cochrane Organization, “Cochrane Reviews are systematic reviews of primary research in human health care and health policy, and are internationally recognised as the highest standard in evidence-based health care. They investigate the effects of interventions for prevention, treatment and rehabilitation.  Every study included in the review is carefully assessed for bias, and given a quality rating score. Cochrane reviews follow a very specific protocol, and the one you develop has to be accepted by the Cochrane Organization before you can start the review:

http://www.cochranelibrary.com/cochrane-database-of-systematic-reviews/

Cochrane reviews usually involve the review of randomised controlled trials, but may also include other types of controlled trials. Because of the rigour required, they can take 12 months or more to undertake, often require several researchers to be involved, and are therefore often not suited to the PhD timeframe.

Systematic review  

Standard systematic reviews are still rigorous, but not bound by the rules of the Cochrane Organization. For example, the researcher can choose or even create their own quality rating score for studies. They are again best suited to reviews of controlled trials. The key features of a general systematic review are that explicit and transparent methods are used, and a standard set of stages are undertaken. Students intending to undertake a systematic review should read the PRISMA statement, which provides guidelines for how to report them: http://prisma-statement.org/

A well-conducted systematic review can also take up to 12 months to conduct, so the student should give careful thought as to whether a systematic review is essential.

Scoping review

Scoping reviews are undertaken when feasibility of the research is considered to be a challenge, either because the relevant literature is thought to be vast and diverse and/or it is thought that little literature exists. In the scoping review, the same systematic, rigorous methods used by the systematic review are used to find studies and extract data. Analyses and syntheses are part of the scoping review, but the depth and type of analysis are different. Scoping reviews are ideal for a PhD in that they are still rigorous, but take a lot less time to undertake than systematic reviews. Here is a good reference:

http://ktdrr.org/products/update/v4n1/dijkers_ktupdate_v4n1_12-15.pdf

Integrative review

Integrative reviews are very similar to scoping reviews, and are the broadest type of research review method. They allow for the inclusion of experimental and non-experimental research, qualitative studies, and can also include theoretical papers. They are most commonly used in nursing research. Here is a reference to undertaking an integrative review:

http://www.aornjournal.org/article/S0001-2092(06)62653-7/fulltext

Narrative review

These are the least rigorous of the various types of literature review. The researcher still needs to describe the databases searched and the search terms used, but there is no formal attempt to rate papers on their quality. Instead, the researcher addresses the different studies in a narrative fashion, comparing and contrasting them as required. Because they lack rigour compared to the other types of literature review, it is more difficult to get them published.  None-the-less, they are still very commonly found in theses. Here is a reference to narrative reviews.

http://www.ease.org.uk/sites/default/files/writing-reviews.pdf

What type of literature review should you do?

The choice depends very much on the topic and what literature is out there. Is it mainly controlled trials, or a mixture of quantitative and qualitative research?  Whichever type of literature review you undertake, there are two golden rules.

Firstly, start broad and work towards the specific topic of interest. Think of the review as an upside down triangle. In the example of protective helmets for AFL players, you might start by looking at sports injuries in general, concussion as a result of playing a sport, the use of protective helmets in sports, concussion in AFL players, and finally, protective helmets for AFL players.

Secondly, there is only one main reason for undertaking a literature review as part of a thesis, and that is to justify why the research needs to be done. Make sure the final paragraph of your literature review reflects this.