Date: Wed, 8 Dec 1999 15:00:25 0500
ReplyTo: Alan Neustadtl <aneustadtl@socy.umd.edu>
Sender: "SAS(r) Discussion" <SASL@LISTSERV.UGA.EDU>
From: Alan Neustadtl <aneustadtl@SOCY.UMD.EDU>
Subject: Achieved Probability Explanation
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I have thought myself into a corner and hope someone can set me
straight. In proc ttest, SAS presents the achieved probability of the
absolute value of the calculated t or greater. The manual explains
that this is presented as a 2tailed test. In other words, you can
directly compare your alpha with the achieved probability. If alpha ls
less than the achieved probability, you fail to reject the null in a
2tailed test. If alpha is greater, you reject the null.
To perform a 1tailed test, you should cut the achieved probability in
half and compare your alpha with this value and make a decision similar
to the one above if alpha is greater than p, reject the null
hypothesis; if alpha is less than p, fail to reject the null
hypothesis.
To make this concrete, assume a large sample that produces a calculated
t value of 1.96, and alpha equal to 0.05. SAS reports Prob>T equal
to 0.0500. Admittedly, this is at the borders, but for a 2tailed
test, let's say that is is close enough that you fail to reject the
null hypothesis. Alternatively, for a
1tailed test, you would compare 0.05 to 0.025 and would reject the
null hypothesis. I understand that this produces results as expected,
but I can't for the life of me grasp the underlying concept of why?
Any help is appreciated, and I apologize at th eoutset for being so
muddled!
Best,
Alan
