Humans who participate in a research study are properly called research subjects
After reading a discussion section of a research article, asking if there are alternative explanations for the results is a good way to come up with an idea for a future study.
primary sources contain descriptions of research by those who conducted it
Jean Piaget observed his own children and developed a theory about the cognitive development of all children. What kind of reasoning was that?
A problem with primary sources is that they may provide an incomplete or misinterpreted description of a research study or a research result.
A good way to come up with a research idea is to look in the discussion section of a research article, and find the discussion of ideas for future research.
A literature search is successfully ended when you stop uncovering new leads.