Queries regarding ANCOVA and MANCOVA

#24
Lol :D that was great! I have already forgotten the existence of beings other than virtual ones. Today I was looking for one of my kittens which was ill. After few seconds, I find myself urged to press control+F keys!!!

Another day, I was reading a paper book. Then the urge to hit the control+F keys emerged frequently! :)

Computer (and technology) have slaved us all. (and the bots living in the cyberspace might as well) :D

But can't we trick SPSS by putting the covariates in the model and running the post hocs? I mean once we can run the MANCOVA as is, and then put those covariates into the model and run the Bonferroni. I wonder how that would affect the reliability of the result. I know it is not correct, but it is the only choice after pencil/paper, and looking for another good statistical package.

I am currently testing this on other software.
 

Dragan

Super Moderator
#25
If the overall F-test is significant, I would use the Fisher's LSD instead of the Bonferonni adjustment - which is also available. (Of course, we would need to consider the number of adjusted means.)
 
#26
I just wanted to fool SPSS by adding the single line

Code:
/POSTHOC=Var1 Var2 Var3(TUKEY LSD BONFERRONI)
to the MANCOVA code. It ran nicely, but ignored the post hocs, warning:

Code:
The POSTHOC subcommand will be ignored because there are covariates in the design.
 

Dason

Ambassador to the humans
#31
And with GUIs come limitations. I think you're seeing that for yourself with this whole SPSS thing. With R if you can dream it then you can do it.
 

Dragan

Super Moderator
#32
But the LSD is as well deactivated in SPSS.
Well, no, it's not, victor....What you want to do is go to options and bring the grouping variable over into "Display Means for:" and then click the box for "Compare Main effects" and your options are LSD, Bonferroni, and Sidak.
 
#33
Well, no, it's not, victor....What you want to do is go to options and bring the grouping variable over into "Display Means for:" and then click the box for "Compare Main effects" and your options are LSD, Bonferroni, and Sidak.
That was one good hint Dragan. Thanks :)

So why did you mention the deactivation of the post hocs in the first place? And why SPSS developers have done this incosnsitent path to first disallow you to do something, and then provide you with the disallowed thing in a hidden place?!

I see its subcommand changes to the below code. meaning that SPSS considers it something different from a simple post hoc test.

Code:
/EMMEANS=TABLES(Var1) WITH(Var2=MEAN) COMPARE ADJ(LSD)
 

Dragan

Super Moderator
#34
That was one good hint Dragan. Thanks :)

So why did you mention the deactivation of the post hocs in the first place? And why SPSS developers have done this incosnsitent path to first disallow you to do something, and then provide you with the disallowed thing in a hidden place?!

I see its subcommand changes to the below code. meaning that SPSS considers it something different from a simple post hoc test.
I mentioned it because a lot of people don't understand that the usual post-hoc ANOVA tests e.g. Tukey, etc. are not appropriate for ANCOVA because the standard error is not computed correctly.

Briefly, in the denominator of the t-statistic for comparing adjusted means for ANCOVA, the standard error is;

Sqrt [MSw [ 1/ni + 1/nj + [ (Xbar_i - Xbar_j)^2 / SSres(X) ] ]

where MSw is the adjusted mean squares within from the overall analysis and X is the covariate.
 
#35
I mentioned it because a lot of people don't understand that the usual post-hoc ANOVA tests e.g. Tukey, etc. are not appropriate for ANCOVA because the standard error is not computed correctly.

Briefly, in the denominator of the t-statistic for comparing adjusted means for ANCOVA, the standard error is;

Sqrt [MSw [ 1/ni + 1/nj + [ (Xbar_i - Xbar_j)^2 / SSres(X) ] ]

where MSw is the adjusted mean squares within from the overall analysis and X is the covariate.
Thanks a lot Dragan :) Then we have two quite different post hocs but with the same name (for example Bonferroni). One is usable for ANOVA, and the other one which can take the covariate into account is useful for ANCOVA. (and I couldn't understand the rest of the technical notes, but it is good to digg it some day when ready) :)
 

Dragan

Super Moderator
#37
Thanks to all of you...victorxstc, Dason, Dragan..

I will try what Dragan told.

Dragan, why do you prefer LSD over Bonferroni?

I prefer it (LSD) because the Bonferroni adjustment is too conservative. Note that the omnibus F-test should be significant to use the LSD approach.
 
#38
I prefer it (LSD) because the Bonferroni adjustment is too conservative. Note that the omnibus F-test should be significant to use the LSD approach.
ok.

I think I need to go through all these post hoc procedures in detail.
Can you please provide me with link (if you have) having the step wise procedure for ANCOVA/MANCOVA and the post hoc procedures?
 
#39
For doing ANCOVA go through "Menu toolbar -> Analyze -> General Linera Model -> Univariate" then hit OK so that the GUI starts. In the appeared dialog box, select your dependent variable and insert it using the arrows into the box labeled "Dependent variable". Then put your independent variables into the one of the two "Factors" boxes. Then put your covariates into the "Covariate" boxes.

Now hit the button labeled "Options". Drag or insert your desired variables into a box labeled "Display means for".

Then check the option "Compare main effects" below that box.

Then from the drop-down menu below that box and that checked option, select one of the options "Bonferroni, LSD, or Sidak".
 
#40
For doing MANCOVA, do the same. But when starting, instead of the path "Menu toolbar -> Analyze -> General Linera Model -> Univariate", go through the path "Menu toolbar -> Analyze -> General Linera Model -> Multivariate". The rest is the same. Note that in this dialog box, you can put more than one variable in the "dependent variable" box.

Here is the step by step instruction to do MANOVA and MANCOVA:
http://www.ucdenver.edu/academics/colleges/nursing/Documents/PDF/MANOVAHowTo.pdf

Note that ANCOVA is similar. But the dialog box opens from the "Univariate" option (instead of Multivariate).