David Airey <[email protected]> asks:
> I guess I would have expected the command syntax to then be:
>
> . anova score noise/subject|noise period/period*subject|noise
> noise*period/perio
> > d*subject|noise dial/dial*subject|noise noise*dial/dial*subject|noise
> period*d
> > ial noise*period*dial, repeated(period dial)
> repeated term
> r(141);
>
> which fails,
>
> rather than:
>
> . anova score noise / subject|noise period noise*period /
> period*subject|noise
> dial noise*dial / dial*subject|noise period*dial noise*period*dial ,
> repeated(period dial)
>
> I thought when a term did not specify the denominator, the residual was
> used. In the latter command, the terms period and dial are listed
> without a denominator so I thought they were using the residual. But
> when I look at the output, it's clear they are using a different (and
> correct) denominator. What am I not understanding in telling or not
> telling Stata what I want?
And then David says:
> OK.
>
> Now I think I understand the syntax, and why Ken's command does what it
> correctly does. All terms prior to the slash without specified
> denominators are tested with the term after the slash. This is a little
> cryptic and depends on correctly grouping terms tested with the same
> denominator when issuing an ANOVA command. But it makes sense once you
> see it. I think the slash is like a shorthand for the lines used to
> separate the ANOVA tables
>
> ANOVA y a b c/d tests each of a, b, and c, over d.
> ANOVA y a/ b c/d tests a over error, and b and c over d
> ANOVA y a/d b/d c/d is disallowed syntax
>
> Right?
"[R] anova" page 43 (Version 8 manual) discusses the "/"
notation.
I admit that there may be better ways to word that discussion.
If anyone has some candidate better wording, send it my way. I
had hoped that the various nested and mixed models that followed
in "[R] anova" would have been enough to clarify.
For your 3 examples above:
> ANOVA y a b c/d tests each of a, b, and c, over d.
Yes.
> ANOVA y a/ b c/d tests a over error, and b and c over d
No. a is tested by b. b is not tested. c is tested by d.
> ANOVA y a/d b/d c/d is disallowed syntax
It is disallowed because the term "d" appears multiple times in
the model, not because of the slashes.
First and foremost the -anova- command is supposed to indicate
which terms are in the model. Secondarily, the "/" notation is
helpful in getting many (often all) of the tests of interest when
the model is written in standard order.
You can get any other tests you want using -test-. When you test
terms you can use the "/" to indicate the error term. See the
examples on pages 60-62 of "[R] anova".
For an example of this in a complicated ANOVA see
http://www.stata.com/support/faqs/stat/anova2.html#expand911
which is one example in a large FAQ filled with examples. In
this case, the "/" in -anova- obtained all but 3 of the tests of
interest. The -test- command using "/" was used to get those 3.
The -anova- command that was typed for that example was:
anova res A / G|A B B*A / B*G|A / S|B*G|A C C*A / C*G|A C*B C*B*A /
C*B*G|A / C*S|B*G|A D D*A / D*G|A D*B D*B*A / D*B*G|A / D*S|B*G|A D*C
D*C*A / D*C*G|A D*C*B D*C*B*A / D*C*B*G|A / , repeated(C D)
Imagine if instead of using the current "/" notation you had to
specify the error term for each instance. It would be even more
complicated, and I contend even harder to understand.
One approach, if you dislike the "/" notation in -anova- is to
not use the "/", and instead do all tests of interest using the
-test- command after -anova-. That way, one by one you can
specify the term to be tested and the error term for that test.
Once you get used to the "/" notation, I am betting you will want
to use it again.
Ken Higbee [email protected]
StataCorp 1-800-STATAPC
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