2010/3/14 Dejian Zhao <zhaodj@ioz.ac.cn>

> To provide more details about the confusing results mentioned in my
> previous email. Some parameters (eg. Sum Sq, Mean Sq, F value, Pr) about the
> two variables seem to depend on the order in the formula, and the variation
> of probability (Pr) directly changes the significance. As to my own data,
> changing the variables order in the formula leads to changes from
> significance to non-significance for one variable.
>

Yes, this is correct, and indeed is part of the point of the example.

The sequential sum of squares depends on the sequence.
The ANOVA table for
ancova(Sodium ~ Calories + Type, data=hotdog)
allows you to test for the effect of Type conditional on Calories already in
the model.

ancova(Sodium ~ Type + Calories, data=hotdog)
allows you to test for the effect of Calories conditional on Type already in
the model.

The Graph, the Residual Sum of Squares, the residuals, and the predicted
values depend on the
entire model.  But the individual rows of the ANOVA table depend on the
order in which
the variables were brought into the model.

Look up "sequential sum of squares" in any intermediate statistics text.
I will recommend mine http://springeronline.com/0-387-40270-5

Rich

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