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A literature review is both a summary and explanation of the current state of knowledge on a focused topic as found in academic books, journal articles and all other sources connected to the subject of study. Its aim is to provide an overview of sources you have explored while researching a particular topic and to demonstrate to your readers how your research fits within the larger field of study.
A literature review must:
be organized around the research question you are developing
cohesively summarize results of what is and is not known
identify gaps in the available literature
formulate questions that you feel needs further research