Google Scholar has it's place in research. That being said, GS is not great for doing a systematic review. The filters are not good, you can't build an advance search, and the Google algorithm is unknown. It can be used for gray literature searching through. Do a simplified search in GS and use the first 300 results or the first few pages of results.
What is gray literature?
Gray literature is defined by the Cochrane Handbook for Systematic Reviews of Interventions as "...literature that is not formally published in sources such as books or journal articles." This can include information such as government reports, conference proceedings, graduate dissertations, unpublished clinical trials, and much more. The sources you select will be informed by your research question and field of study, but should likely include, at a minimum, theses and dissertations.
Why search the gray literature if it hasn't been peer-reviewed?
The intent of an evidence synthesis is to synthesize all available evidence that is applicable to your research question.
There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect. Meanwhile, many studies and trials that show no effect end up going unpublished. But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making. Thus, the gray literature can be critical.
How do I search the gray literature if it's unpublished and not in the scholarly databases?
Its true--finding gray literature and searching it systematically is challenging. But there are a few approaches that you can take to add some structure to your search of this type of information:
Googling the Greys: Tips for Searching Beyond Health Databases and Turning Information into Insights
A thorough grey literature search should involve a general sweep of the web by using different search engines. Google is an important search engine but other search engines, such as Yahoo and Bing, could also be useful.
While Google is a powerful tool for searching for grey literature it should not be used exclusively - other sources should also be searched in order to find grey literature.
1) Restrict content to .org or .gov sites
Type in your topic and then either "site:.org" OR "site:.gov"
2) Restrict content to file type
Type in your topic and then "filetype:pdf" OR "filetype:doc"
3) Use Google Australia, Google UK, etc.
4) Use the Duck Duck Go search engine which does not record location or user searches
5) Use Link Klipper (Chrome extension) to pull results into a Microsoft Excel spreadsheet. You can also use the Grey Literature Search Log form linked below to keep track of your searches: