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Back to table of contents Previous article Next article Communications and UpdatesFull AccessResponse to Irion et al. LetterYvette Sheline, M.D.Yvette ShelineSearch for more papers by this author, M.D.Published Online:1 Apr 2013https://doi.org/10.1176/appi.ajp.2013.12121544rAboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail To the Editor: In their letter, Irion et al. express concern about the volumetric differences reported, stating that they are below the resolution of MRI. However, we would like to bring to their attention basic statistical principles applied in these circumstances (1), in which examination of group differences depends on the statistical properties of the sample, not on resolution in individual scans. The significance of group differences is based on the sample size, the mean group difference, and the standard deviation of the sample. Thus, even small differences can be significantly different if the measurements are repeatable (with a small standard deviation) as is the case in FreeSurfer segmentation, and if there is a relatively large sample size (N=168 in our study) and a sufficiently large difference in the means. In fact, the total volume difference measured was not small: a mean of 0.3 mm3/person×168=50.4 mm3.From the Department of Psychiatry and Radiological Sciences, Washington University School of Medicine, St. Louis.The author’s disclosures accompany the original article.Reference1 Zar JH: Two-sample hypotheses, inBiostatistical Analysis, 4th ed. Upper Saddle River, NJ, Prentice Hall, 1999Google Scholar FiguresReferencesCited byDetailsCited ByNone Volume 170Issue 4 April 2013Pages 446a-446a Metrics PDF download History Accepted 1 February 2013 Published online 1 April 2013 Published in print 1 April 2013
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