medical research studies
You are right, not at the same time. I am looking at income received in 2005 from multiple, short-term studies.
Have you ever looked into participating in a study? Some are long-term (6-12mos), but most are much shorter, such as determining if a new pain med is effective against dental pain. For a study like this, you must already require dental work (like wisdom tooth extraction). You have your tooth removed for free and you receive a small stipend to cover transportation costs - like $50. IF the med does NOT work for you, you get additional compensation (maybe as much as $250). This is an example of a 1 day study.
Most studies involving healthy people only run 2-4wks and require multiple office visits, some require overnight stays. Therefore, it is possible to do several studies over the course of a year. It is a popular source of income for students