some time ago, i sent out a note about a handout i had re: ancova. now, in
that handout, i illustrated a very simple case of how ancova might account
for some of the within groups 'error'. in that handout, i showed, near the
end ... some minitab output for the analysis. now, in that output ... the
adjusted SS adds up to MORE than what the simple anova adds too. NOTE: the
dependent measure in the Exp and Cont group example was performance on a
test .. and the covariate was IQ.
the one way shows:
One-way Analysis of Variance
Analysis of Variance
Source DF SS MS F P
Factor 1 252 252 1.54 0.231
Error 18 2949 164
Total 19 3201
and the ancova shows:
Analysis of Variance for TOTY, using Adjusted SS for Tests
Source DF Seq SS Adj SS Adj MS F P
TOTIQ 1 1539.9 2057.9 2057.9 39.26 0.000
Group 1 770.0 770.0 770.0 14.69 0.001
Error 17 891.0 891.0 52.4
Total 19 3200.9
in the handout, i showed that the adjusted SS(TOT) equals the sum of the
770 and 891 values for Group and Error in the Adj SS columns ... but where
does the 2057 come from and, when you add to the 770 and 891 values .. you
get a much larger value than the original 3201?
what would be the simplest way to discuss this with students? in what way
could you use the original data on the dependent measure ... and show how
this new SS(TOT) value could be obtained?
thanks
----------------------------------------------
208 Cedar Bldg., University Park, PA 16802
AC 814-863-2401 Email mailto:[EMAIL PROTECTED]
WWW: http://roberts.ed.psu.edu/users/droberts/drober~1.htm
FAX: AC 814-863-1002
