The Evidence Hierarchy

Not all evidence is created equal. From strongest to weakest:

• Systematic reviews and meta-analyses — many studies combined
• Randomized controlled trials — gold standard for cause and effect
• Observational studies — find correlations, cannot prove causation
• Expert opinion — useful, but can be biased or outdated
• Anecdotal evidence — personal stories; the weakest form

Most claims you meet daily rest on anecdotes at best. "My friend tried this diet and lost 20 pounds" is one person's experience, which tells you almost nothing about whether the diet works in general. The plural of anecdote is not data.