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st: discrepancy in -sdtest- ?


From   "VISINTAINER PAUL" <PAUL_VISINTAINER@nymc.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: discrepancy in -sdtest- ?
Date   Mon, 22 Nov 2004 12:48:55 -0500

One of my faculty brought this discrepancy to my attention.  Why is the
2-sided test of significance (.8004) not equal to twice the one-sided
test? (e.g., .3749 * 2 = .7498).  She tells me that SAS gives a
two-sided p-value of .7498.

Thanks,
Paul



. sdtesti 9 . 4.849227 16 . 4.475247

Variance ratio test

------------------------------------------------------------------------
------
         |     Obs        Mean    Std. Err.   Std. Dev.   [95% Conf.
Interval]
---------+--------------------------------------------------------------
------
       x |       9           .    1.616409    4.849227           .
.
       y |      16           .    1.118812    4.475247           .
.
---------+--------------------------------------------------------------
------
combined |      25           .           .           .           .
.
------------------------------------------------------------------------
------

                             Ho: sd(x) = sd(y) 

               F(8,15) observed   = F_obs           =    1.174
               F(8,15) lower tail = F_L   = 1/F_obs =    0.852
               F(8,15) upper tail = F_U   = F_obs   =    1.174

   Ha: sd(x) < sd(y)         Ha: sd(x) != sd(y)          Ha: sd(x) >
sd(y)
  P < F_obs = 0.6251     P < F_L + P > F_U = 0.8004     P > F_obs =
0.3749




______________________________
Paul F. Visintainer, PhD
Professor and Program Director 
Health Quantitative Sciences
School of Public Health
New York Medical College 



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