Research shows that many students do not know how to paraphrase. They rearrange the words, instead of synthesizing and summarizing the concepts. Inadequate paraphrasing is the main cause of plagiarism.
Many students are not aware that even if they paraphase they must give credit to the originator of the ideas. To put it another way, even if you do not quote someone, you must still cite them if you use their thoughts.
One article reporting such research is by Pamela A. Jackson. "Plagiarism Instruction Online: Assessing Undergraduate Students' Abillity to Avoid Plagiarism." College and Research Libraries 67, no. 5 (September 2006): 418-428.
The article's author was involved with creating an online tutorial on plagiarism and paraphrasing. You can access this tutorial at http://tutorials.sjlibrary.org/tutorial/plagiarism/selector.htm and brush up on your skills!