hey there,

a stressed undergrad here trying to get to grips with ANCOVA. One of the advantages of ANCOVA versus ANOVA is that it reduces the error term and thus increases power. If you do a regular ANOVA, you get the F value. If you then include a covariate and perform ANCOVA - this F value is reduced. How is it reduced (especially if the error term is being reduced)? I guess I don't understand the stats workings behind ANCOVA and what its doing to eliminate the covariate.

Any help would be greatly appreciated as I have an exam next week

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Just noticed that the example I'm using in spss is where the means of the covariate are different for 3 levels of the IV. when I change this in spss so that the means of the CV are the same for all IV levels, the F value increases (compared to regular ANOVA). But I still don't get what calculations spss is doing to remove the CV and how differences in the CV mean affect this?? My text book doesn't cover ANCOVA, my lecturers notes don't go into this detail and I just can't understand explanations of ANCOVA from the net:-/

help?

cheers,

Karen

a stressed undergrad here trying to get to grips with ANCOVA. One of the advantages of ANCOVA versus ANOVA is that it reduces the error term and thus increases power. If you do a regular ANOVA, you get the F value. If you then include a covariate and perform ANCOVA - this F value is reduced. How is it reduced (especially if the error term is being reduced)? I guess I don't understand the stats workings behind ANCOVA and what its doing to eliminate the covariate.

Any help would be greatly appreciated as I have an exam next week

----------------------------

Just noticed that the example I'm using in spss is where the means of the covariate are different for 3 levels of the IV. when I change this in spss so that the means of the CV are the same for all IV levels, the F value increases (compared to regular ANOVA). But I still don't get what calculations spss is doing to remove the CV and how differences in the CV mean affect this?? My text book doesn't cover ANCOVA, my lecturers notes don't go into this detail and I just can't understand explanations of ANCOVA from the net:-/

help?

cheers,

Karen

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