Test-Retest Reliability
A measure of how consistent a test's results are when the same person takes it again. High reliability (0.70+) means you'll get similar results each time.
Test-retest reliability is one of the most important quality metrics for personality tests. It answers the question: "If I take this test again next month, will I get the same result?"
Reliability is measured as a correlation coefficient from 0 to 1. Above 0.70 is considered acceptable; above 0.80 is good; above 0.90 is excellent. The Big Five has reliability of 0.75-0.90 (good to excellent). MBTI has approximately 0.50 reliability — meaning about 50% of people get a different 4-letter type when retaking. Short social media quizzes (5-10 questions) typically have reliability below 0.50.
This is why JobCannon uses 30-60 questions per test — more questions increase reliability. It's also why Big Five scores (continuous percentiles) are more reliable than MBTI types (binary categories that flip near the midpoint).