Reliability

What is Reliability?

Reliability refers to the consistency and stability of a test score. If you step on a scale and it says 70kg, then step off and on again and it says 85kg, the scale is unreliable.

In IQ testing, reliability is crucial. If you take an IQ test on Monday and score 130, and then take it again on Friday and score 100, the test is useless.

Test-Retest Reliability

The gold standard for measuring this is test-retest reliability. Professional tests like the WAIS-IV have extremely high reliability coefficients (often above 0.95), meaning if you take the test twice (with enough time in between to forget specific answers), your score will likely be within a few points of the original.

Sources of Error

Even the best tests aren’t perfect. Reliability can be affected by:

• Internal State: Sleep deprivation, anxiety, or illness can temporarily lower a score.
• Environment: A noisy room or distracting proctor.
• Guessing: Multiple-choice formats introduce a small element of luck.

Reliability vs. Validity

It is possible for a test to be reliable but not valid.

• Example: If you measure intelligence by measuring head circumference with a tape measure, you will get the exact same number every time (High Reliability), but it tells you nothing about intelligence (Low Validity).