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st: Anova and pwcompare and not estimable
From
Garry Anderson <[email protected]>
To
"[email protected]" <[email protected]>
Subject
st: Anova and pwcompare and not estimable
Date
Mon, 9 Jan 2012 08:52:34 +0000
Dear Statalist,
The following output from the -pwcompare- command after -anova- compares all pairwise differences in a repeated measures anova with one factor being repeated. The standard errors are not estimable when comparing levels of the between subject factor (calib). There do not appear to be any empty cells. Empty cells can cause the 'not estimable' text.
Calib is a between subject factor and shape is a within subject factor.
Note I have separate subjects for the two levels of calib.
An suggestions as to how to estimate these standard errors would be appreciated.
An example follows
. set more off
. use http://www.stata-press.com/data/r12/t77
(T7.7 -- Winer, Brown, Michels)
. gen sub1to6 = subject
. replace sub1to6=sub1to6 + 3 if calib==2
(12 real changes made)
. anova score calib / sub1to6|calib shape calib#shape,repeated(shape)
Number of obs = 24 R-squared = 0.8925
Root MSE = 1.11181 Adj R-squared = 0.7939
Source | Partial SS df MS F Prob > F
--------------+----------------------------------------------------
Model | 123.125 11 11.1931818 9.06 0.0003
|
calib | 51.0416667 1 51.0416667 11.89 0.0261
sub1to6|calib | 17.1666667 4 4.29166667
--------------+----------------------------------------------------
shape | 47.4583333 3 15.8194444 12.80 0.0005
calib#shape | 7.45833333 3 2.48611111 2.01 0.1662
|
Residual | 14.8333333 12 1.23611111
--------------+----------------------------------------------------
Total | 137.958333 23 5.99818841
Between-subjects error term: sub1to6|calib
Levels: 6 (4 df)
Lowest b.s.e. variable: sub1to6
Covariance pooled over: calib (for repeated variable)
Repeated variable: shape
Huynh-Feldt epsilon = 0.8483
Greenhouse-Geisser epsilon = 0.4751
Box's conservative epsilon = 0.3333
------------ Prob > F ------------
Source | df F Regular H-F G-G Box
--------------+----------------------------------------------------
shape | 3 12.80 0.0005 0.0011 0.0099 0.0232
calib#shape | 3 2.01 0.1662 0.1791 0.2152 0.2291
Residual | 12
-------------------------------------------------------------------
. pwcompare calib#shape
Pairwise comparisons of marginal linear predictions
Margins : asbalanced
-----------------------------------------------------------------
| Unadjusted
| Contrast Std. Err. [95% Conf. Interval]
----------------+------------------------------------------------
calib#shape |
(1 2) vs (1 1) | -1 .9077853 -2.977894 .9778942
(1 3) vs (1 1) | 3 .9077853 1.022106 4.977894
(1 4) vs (1 1) | .6666667 .9077853 -1.311227 2.644561
(2 1) vs (1 1) | . (not estimable)
(2 2) vs (1 1) | . (not estimable)
(2 3) vs (1 1) | . (not estimable)
(2 4) vs (1 1) | . (not estimable)
(1 3) vs (1 2) | 4 .9077853 2.022106 5.977894
(1 4) vs (1 2) | 1.666667 .9077853 -.3112275 3.644561
(2 1) vs (1 2) | . (not estimable)
(2 2) vs (1 2) | . (not estimable)
(2 3) vs (1 2) | . (not estimable)
(2 4) vs (1 2) | . (not estimable)
(1 4) vs (1 3) | -2.333333 .9077853 -4.311227 -.3554392
(2 1) vs (1 3) | . (not estimable)
(2 2) vs (1 3) | . (not estimable)
(2 3) vs (1 3) | . (not estimable)
(2 4) vs (1 3) | . (not estimable)
(2 1) vs (1 4) | . (not estimable)
(2 2) vs (1 4) | . (not estimable)
(2 3) vs (1 4) | . (not estimable)
(2 4) vs (1 4) | . (not estimable)
(2 2) vs (2 1) | -1.666667 .9077853 -3.644561 .3112275
(2 3) vs (2 1) | 1.666667 .9077853 -.3112275 3.644561
(2 4) vs (2 1) | 2.333333 .9077853 .3554392 4.311227
(2 3) vs (2 2) | 3.333333 .9077853 1.355439 5.311227
(2 4) vs (2 2) | 4 .9077853 2.022106 5.977894
(2 4) vs (2 3) | .6666667 .9077853 -1.311227 2.644561
-----------------------------------------------------------------
. table calib shape,con(n score)
----------------------------------
2 methods |
for |
calibrati | 4 dial shapes
ng dials | 1 2 3 4
----------+-----------------------
1 | 3 3 3 3
2 | 3 3 3 3
----------------------------------
.
Kind regards, Garry
Garry Anderson
Faculty of Veterinary Science
University of Melbourne
250 Princes Highway Ph 03 9731 2221
WERRIBEE 3030 Fax 03 9731 2388
Email: [email protected]
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