Date: Thu, 6 Sep 2007 11:55:52 -0700 Reply-To: Paige Miller Sender: "SAS(r) Discussion" From: Paige Miller Organization: http://groups.google.com Subject: PROC NPAR1WAY One-Sided Tests Comments: To: sas-l@uga.edu Content-Type: text/plain; charset="iso-8859-1" I am confused by the description of the one-sided tests in PROC NPAR1WAY for the case where we compare two means via simple rank tests. When I learned statistics, you specified a null hypothesis (in this case, that the means of two groups are equal) and an alternative hypothesis. If the alternative hypothesis was that the means of two groups were not equal, then you had a two-sided test. If the alternative hypothesis was that group 1 mean was less than group 2 mean, you had a one-sided test. There is another one-sided test that can be specified in the opposite direction by having the alternative hypothesis be that group 2 mean is less than the group 1 mean. However, although PROC NPAR1WAY reports one-sided tests, it doesn't let you specify which direction the alternative hypothesis points. The documentation for PROC NPAR1WAY very unhelpfully explains that the direction of the test depends on whether the computed statistic z is greater than zero, or not. This makes no sense to me. The direction of the alternative hypothesis does not depend upon the computed statistic. Even forgetting that, they do not specify what alternate hypothesis is tested when z>0, and I am reluctant to assume that the alternate hypothesis tested is that mean of group 1 > mean or group 2 or vice versa, because I don't know how they pick which level is group 1, and they don't say that when z>0 they are testing group1 mean > group2 mean (as it could be that they are testing group2 mean > group1 mean when z>0). Someone please explain this to me. Thanks. -- Paige Miller paige\dot\miller \at\ kodak\dot\com 

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