Statistics How To

Test-Retest Reliability / Repeatability

Statistics Definitions > Test-Retest Reliability / Repeatability

Test-Retest Reliability

Test-retest reliability

Test reliability is measured with a test-retest correlation.

Test-Retest Reliability (sometimes called retest reliability) measures test consistency — the reliability of a test measured over time. In other words, give the same test twice to the same people at different times to see if the scores are the same. For example, test on a Monday, then again the following Monday. The two scores are then correlated.

Bias is a known problem with this type of reliability test, due to:

  • Feedback between tests,
  • Participants gaining knowledge about the purpose of the test, so they are more prepared the second time around.

This reliability test can also take a long time to calculate correlations for. Depending upon the length of time between the two tests, this could be months or even years.

Calculating Test-Retest Reliability Coefficients

Finding a correlation coefficient for the two sets of data is one of the most common ways to find a correlation between the two tests. Test-retest reliability coefficients (also called coefficients of stability) vary between 0 and 1, where:

  • 1 : perfect reliability,
  • ≥ 0.9: excellent reliability,
  • ≥ 0.8 < 0.9: good reliability,
  • ≥ 0.7 < 0.8: acceptable reliability,
  • ≥ 0.6 < 0.7: questionable reliability,
  • ≥ 0.5 < 0.6: poor reliability,
  • < 0.5: unacceptable reliability,
  • 0: no reliability.

On this scale, a correlation of .9(90%) would indicate a very high correlation (good reliability) and a value of 10% a very low one (poor reliability).

  • For measuring reliability for two tests, use the Pearson Correlation Coefficient. One disadvantage: it overestimates the true relationship for small samples (under 15).
  • If you have more than two tests, use Intraclass Correlation. This can also be used for two tests, and has the advantage it doesn’t overestimate relationships for small samples. However, it is more challenging to calculate, compared to the simplicity of Pearson’s.

Need help with a homework or test question? With Chegg Study, you can get step-by-step solutions to your questions from an expert in the field. If you rather get 1:1 study help, Chegg Tutors offers 30 minutes of free tutoring to new users, so you can try them out before committing to a subscription.

If you prefer an online interactive environment to learn R and statistics, this free R Tutorial by Datacamp is a great way to get started. If you're are somewhat comfortable with R and are interested in going deeper into Statistics, try this Statistics with R track.

Comments? Need to post a correction? Please post a comment on our Facebook page.

Check out our updated Privacy policy and Cookie Policy

Test-Retest Reliability / Repeatability was last modified: November 13th, 2017 by Stephanie