If you use a ruler to measure a table, you also measure the ruler by the table.
Taleb's formulation. If a test's error rate exceeds the base rate of what it detects, a positive result tells you more about the test than about the subject.
In practice
- A tape measure of unknown accuracy laid against a table tells you as much about the tape as about the table.
- A screening tool that flags 3% of staff as risks in a population where 0.1% are risks is measuring the tool.