3.1. Material collection
The material of the literature review and the unit of analysis are detailed in this part. The study was conducted from December 2012 to May 2013 covering the accepted papers (available online) in scientific English language journals from January 2007 to March 2013. The search procedure was managed in three stages with the “reverse logistics and closed loop supply chain” keywords in the Google-scholar search engine (www.scholar.google.com) with these modifications: searching for articles in English language, and custom time range between 2007 and 2013, sorted by relevance. It should be mentioned that the search engine is updated periodically due to the acquisition of new publications, relevance, citations, and so forth, so the process of collecting papers is undertaken in a short period of time. The three stages of the research procedure are as follows: