An author of a peer reviewed article can handle data in a couple ways. I have examples below.
1) Primary research: this is when the researchers who are authoring the article gather data themselves- maybe through a survey, poll or study.
Here's how to know if data is collected by the authors (primary research): When the authors describe how they designed the study or data gathering, that means they did it themselves:
2) Secondary analysis (of existing data, such as from a government, non-profit research institutions and business entities): Researchers use existing data that they couldn't gather on their own. Gathering, organizing and storing data is expensive. The US government plays an essential role in gathering and providing free access to data, such as demographics. The business world gathers data for the purpose of marketing - but this kind of data is not shared for free.
Here are two examples of how to trace secondary data within a paper - these are the two places I would check to quickly get an idea of where the authors got their data. They will also cite their data in the references section at the end. Open in new tab/window to view larger.
1) In the methods section of the paper:
2) Under a table or graph of data:
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