A tail of two sides: Artificially doubled false positive rates in neuroimaging due to the sidedness choice with t-tests

Hum Brain Mapp. 2019 Feb 15;40(3):1037-1043. doi: 10.1002/hbm.24399. Epub 2018 Sep 28.


One-sided t-tests are widely used in neuroimaging data analysis. While such a test may be applicable when investigating specific regions and prior information about directionality is present, we argue here that it is often mis-applied, with severe consequences for false positive rate (FPR) control. Conceptually, a pair of one-sided t-tests conducted in tandem (e.g., to test separately for both positive and negative effects), effectively amounts to a two-sided t-test. However, replacing the two-sided test with a pair of one-sided tests without multiple comparisons correction essentially doubles the intended FPR of statements made about the same study; that is, the actual family-wise error (FWE) of results at the whole brain level would be 10% instead of the 5% intended by the researcher. Therefore, we strongly recommend that, unless otherwise explicitly justified, two-sided t-tests be applied instead of two simultaneous one-sided t-tests.

Keywords: false positive rate; family-wise error; null hypothesis significance testing; one- and two-sided testing.

Publication types

  • Research Support, N.I.H., Intramural

MeSH terms

  • Data Interpretation, Statistical*
  • False Positive Reactions*
  • Humans
  • Image Interpretation, Computer-Assisted / methods
  • Neuroimaging / methods*