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Sarah, In SPM, a t-test is typically one-tailed. If you are interested in a two-tailed t-test, you can define the same contrast as an F-test. This is because a square of t-test statistic (df=N) is an F-test statistic (df=1,N). This means both highly negative and highly positive t-scores will manifest as large F-scores. -Satoru From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Pulaski, Sarah (NIH/NINDS) [V] Sent: Monday, November 16, 2009 1:17 PM To: [log in to unmask] Subject: [SPM] One Sample test - one-tailed or two tailed Hello, I was wondering if anyone could help me figure out what is likely, a very simple question. I have been using SPM5 to analyze lesion data in one sample of patients, correlating their brain atrophy with behavioral data. I have set my contrast code as 0 1 0 0... (and so forth). Is this analysis a 1-tailed t-test or a 2-tailed t-test. Is it even possible to do a 2-tailed t-test in SPM? Thanks so much, Sarah Sarah Pulaski, M.A. Cognitive Neuroscience Section National Institute of Neurological Disorders and Stroke Building 10; Room 7D48 MSC 1440 Bethesda, Maryland 20892-1440 USA E-Mail: [log in to unmask] Work Phone: 301-496-8163 Work FAX: 301-480-2909