So you've decided to do a randomized design with pre and a post tests?
Here is model of the design:
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Rt -> Pr -> I -> Po
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Rc -> Pr -> Po
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Rt=Randomized treatment group
Rc=Randomized comparison group
Pr=Pre-test
I=Intervention
Po=Post-test
Analysis of Designs with a Pre and Post Test.
Designs with both pre and post tests must be analyzed using Analysis of Covariance (ANCOVA). What is ANCOVA? ANCOVA is statistical a method that allows you to covary the pretest measurement with the outcome meaurement. ANCOVA is basically a linear regression model that allows you to adjust for the pretest measure. In other words, we are just removing the effect of the pretest measure so that we can just look at the difference between the post test measurements between the treatment and comparison groups.
For example, The ANCOVA model looks like this:
Outcome= intercept + (constant1)Pretest + (constant2)Z[t or c] + Error term
Where Z is used to indicate you treatment verses your comparison group (Z=1 treatment, Z=0 comparison).
Here is an example of an ANCOVA analysis in MINITAB statistical Package
(I made up this data!)
The regression equation is posttest = 0.107 - 0.053 pretest + 0.069 Z Predictor Coef StDev T P Constant 0.1071 0.1466 0.73 0.230 pretest -0.0530 0.1009 -0.53 0.100 Z 0.0687 0.1995 0.34 0.042 S = 0.9942 R-Sq = 0.57 R-Sq(adj) = 56.2%
We can see from the P-values, that P<.05 for the Z predictor, so we know that Z is significant, and therefore,
there is a difference between the treatment and comparison groups.
If you would like more information on ANCOVA, Applied Linear Statistical Models, by Neter, Kutner, Nachshem, and Wasserman is a great reference. This book also discusses P-values, and significance levels if you need a refresher.
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ANCOVA can be done in a variety of statistical packages
The easiest and/or most versatile packages that do ANCOVA are :
1) MINITAB-very user friendly-point & click
2) SAS-sorry, you have to write your own code for this one!
3)SPSS-again,point & click
Please Email me with any information
or comments about this web page
4/9/97