Begin your search in your personal files archive before moving on to electronic databases. First, read their titles and abstracts. Eliminate those studies that do not match with your requirements. Then, for the remaining articles, read their reference lists, retrieve the abstracts of articles you think are important to your research, and continue the process. You should aim to retrieve all relevant articles. You also need to search experts and researchers working in the area of your interest. It’s a good idea to search newsgroups and mailing lists, in addition to contacting individual authors.