A High Quality Systematic Review should seek to
Courtesy of What is a systematic review? By Hemingway, Pippa and Breton, Nic.
For more information see The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate healthcare interventions: explanation and elaboration. By Liberati A; et al.
Before starting a systematic review look at PROSPERO, an International prospective register of systematic reviews hosted by The University of York and the National Institute for Health Research. Checking this can help researchers avoid duplicating current systematic reviews, saving the researcher a lot time and heartache. Registering with PROSPERO is free and allows researchers to notify others of their prospective reviews.
Grey Literature: Grey Literature, may also be spelled Gray Literature, is defined by The Fourth International Conference on Grey Literature as "That which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers."
Put simply, Grey Literature is research that is often not indexed in the databases provided by libraries and is often conducted by government agencies, both foreign and domestic, and NGOs (Non Government Organizations). This information can be difficult to locate, but includes valuable research and should be consulted for systematic literature reviews.
Grey Literature is especially dependent on the topic you are researching, so please contact me at email@example.com to set up a research consultation if you are having trouble locating information or need help deciding where to start.