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Yes. X must be correlated with Y in a mediation analysis. The reason is as follows - if X and Y aren't related, then there is nothing to mediate the relationship. The fact that you can't apply a mediation analysis doesn't preclude you from saying X is correlated with M and M is correlated with Y. There is no direct effect and thus no mediation. There is simply an indirect effect. Most people would then discuss the direct effect of M on Y. Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Postdoctoral Research Fellow, GRECC, Bedford VA Website: http://www.martinos.org/~mclaren Office: (773) 406-2464 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email. On Sun, May 31, 2015 at 1:59 AM, Mike <[log in to unmask]> wrote: > Hi, > > This question may not be directly related to spm but growing studies use > mediation analysis to analyse fmri data. Say that X (indept variable) is > correlated with M (mediator), and M is correlated with Y (dept variable). > If I want to apply mediation analysis, is it a must that X is correlated > with Y? Thanks. Mike >