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Read the methods first, and find where bias enters

The methods section is where critical appraisal happens, so we read it before the abstract and the introduction. The rest of the paper reads differently once we know what the researchers actually did. Six questions structure that read, and together they ask whether the design, the sample, and the measurements can support the paper's claim. The first is the plainest: can we state the study's question in one sentence? Underneath all six sits systematic error — confounding, who ended up in the sample, and the measurements themselves.

Also covered: sampling strategy · inclusion and exclusion criteria · validity and reliability · missing data · statistical adjustment