Evaluating Evidence: Relevant, Sufficient, Credible

Judge whether evidence actually supports a claim, and whether there is enough of it.

Not all evidence is equal. Strong evidence passes three tests. Relevant: does it actually bear on the claim? Sufficient: is there enough of it, and is it representative? Credible: does it come from a reliable source using sound methods? Worked example: a writer argues that a new reading program improves test scores across the state. Evidence offered: 'At Lincoln Middle School, scores rose 12 percent after the program began, and the principal says students love it. ' Relevant? Partly; scores are relevant, but students loving the program is not evidence of improved scores.

Sufficient? No; one school cannot represent a whole state, and other changes may have happened that year. Credible? The principal may have a stake in the program's success. Stronger evidence would include results from many schools, a comparison group that did not use the program, and an independent evaluator. Running these three tests turns a vague sense of doubt into a specific critique.

• 1Relevant: does the evidence bear on the claim?
• 2Sufficient: is there enough, and is it representative?
• 3Credible: is the source reliable and the method sound?