I'm not a teacher but as a former college student I can tell you what a few of my profs did -
After the paper was turned in, usually within a couple of days, we were given a sheet of paper that asked these types of questions:
What did you learn from the research about your paper?
What did you find to be the most challenging aspect of finding sources, documentation,etc?
What did you like about the research?
If someone else had to do this research paper, what sources would you recommend to them?
In your own words please give me three main points from your research.
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Believe me, if anyone in the class did not do their own research they had a lot harder time answering these simple questions than those of us that actually did the research. Without the papers right there to read from the answers were from memory.
The teachers used the answers to the questions along side of the research paper and combined the grade. If a sheet was turned in that looked like the student didn't know anything about their paper or didn't know anything beyond what was on the paper, the teacher would return the paper with a note "please see me in my office to discuss"
during which time I'm sure the student had a chance to defend their work.
I don't know if this helps you at all, but I do know that it worked for the classes I was in. After word gets out that the teacher will spring a quiz on you about your research and so forth, alot of students will kick it in and do their own papers.