Measurement Error
The inconsistency in test scores resulting from imperfect measurement. Every personality test has measurement error — your score is an estimate, not a perfect measure.
No test is perfectly reliable. On a Big Five test, your Extraversion percentile might be 58, but the 95% confidence interval is 53-63. The score is an estimate.
Sources of measurement error: momentary mood (taking the test tired vs. well-rested), question ambiguity (interpreting words differently), response bias (faking), and random variation.
Measurement error is reported as the Standard Error of Measurement (SEM). A test with SEM=5 means your score could be ±5 points from the true score. Higher reliability (higher internal consistency) = lower SEM = more precise scores.
This is why one test score shouldn't change your entire career plan. Multiple assessments (Big Five + RIASEC + Values) over time give a clearer picture.