As research is created and disseminated, it moves from being a primary source → → toward being → → a tertiary source.
The earlier in the process (and closer to the actual science being done) the more likely you are dealing with a primary sources.
When you start to research a topic (before you do any science), you go the opposite direction ← ←. You may start with a tertiary source, like Wikipedia or library database and move toward the original work or primary research.
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