Helga,
Please see below in capital letters.
Good luck.
Dominic Lusinchi
Statistician
Far West Research
Statistical Consulting
San Francisco, California
415-664-3032
www.farwestresearch.com
-----Original Message-----
From: SPSSX(r) Discussion [mailto:
[hidden email]] On Behalf Of
Helga Walz
Sent: Monday, October 23, 2006 6:55 AM
To:
[hidden email]
Subject: your assistance please
Your assistance, please
SPSS provides probabilities for 2-tailed tests only.
With a 1-tailed test and alpha 0.05, I understand that I need to divide
the Sig. (2-tailed).
YES THE P-VALUE MUST BE DIVIDED BY 2.
Thus, if SPSS list the p-value as 0.881, that means for the 1-tailed
test the
p-value is 0.04405.
I THINK YOU MEANT .4405 (A NON-SIGNIFICANT VALUE) IF THE TWO-SIDED P-VALUE
IS .881 (NOT .0881), SO YES THAT IS CORRECT. IF THE TWO-SIDED P-VALUE IS
.0881 THEN .04405 IS CORRECT FOR A ONE-SIDED ALTERNATIVE HYPOTHESIS.
(HOWEVER, SEE RICHARD RISTOW'S COMMENT ABOUT THESE.)
Does the alpha level have to be changed in SPSS from 0-05 to 0.10 for
the same
1-tailed test?
I AM NOT SURE WHAT YOU MEAN HERE. IT WOULD HELP IF YOU DESCRIBED THE TYPE OF
TEST YOU ARE PERFORMING. SPSS COMPUTES THE EXACT P-VALUE FOR ANY TEST - AS
FAR AS I KNOW.
Also, is the 95% Confidence Interval affected at all whenever we conduct
a
1-tailed test, i.e., the lower and upper values?
YES. A CONFIDENCE INTERVAL USES BOTH TAILS OF THE SAMPLING DISTRIBUTION OF
THE STATISTIC OF INTEREST. IF YOU SET YOUR ONE-SIDED SIGNIFICANCE LEVEL TO
.05 YOU ARE LOOKING AT THE AREA UNDER THE CURVE IN ONE TAIL. WHEN YOU
COMPUTE A 95% CONFIDENCE INTERVAL BOTH SIDES OF THE DISTRIBUTION ARE TAKEN
INTO ACCOUNT.
Thank you for your assistance,
Helga Walz
Helga S. Walz, Ph.D.
Division of Applied Behavioral Sciences
University of Baltimore
1420 North Charles Street
Baltimore, Maryland 21201
Tel.: 410 837-5279
Fax.:410 837-4059
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