[FieldTrip] ANOVA and tails

[Gopakumar Venugopalan]

Dear Olga, this does not mean only right tail is possible. You could have a
positive or negative sign for your test statistic, which is acceptable. That
can be fixed in Fieldtrip or EEGLAB by the order you enter Condition1 and
Condition 2. But to answer your question more substantively:
A one-tail test is more consevervative than a two-tail test. A two-tail test
is when you have no a priori expectation where you expect the condition 1 -
condition 2 to be higher or lower. Using a non EEG example if you have two
groups treatment and control, you will expect yoga to lower the depression
rates in treatment group and not the control group.
Similarly if you have two groups treatment and control, you will expect some
protein shake to yield higher muscle mass in the treatment group and not the
control group. In both cases the outcome (negative or lower in scenario one,
higher or positive on scenario two) is an a priori expectation. So in a EEG
sense we know that the Incongruent or deviant word will have a higher
displacement than the congruent or expected word..
Going back to the conservative versus liberal nature of the statistic.
If the tabled value of  the F (df1=1, df2=11) is 4.84 that is the size of
the tail or the reject region, however when you halve that you tabled value
is half of it. So you obtained value in the first case would have to be
greater than 4.85, while in the two-tail case it slide with anything over
2.43. Therefore the two-tail is not only for exploratory purposes, but is
also less conservative.

I hope I have helped.

Warm regards
gopa

On Thu, Sep 1, 2011 at 7:51 AM, Sysoeva, Olga Vladimirovna <
sysoevao at psychiatry.wustl.edu> wrote:

>  Dear Fieldtrippers,****
>
> I’ve tried to use between subject ANOVA (independentF) and a bit confused
> with the tails.****
>
> I’ve got the following message****
>
> “For an independent samples F-statistic, it does not make sense to
> calculate a two-sided critical value.”****
>
> Could you explain me why? Why only right tail is possible? ****
>
> Best Regards,****
>
> Olga.****
>
>
> ____________________________________________________________________________
> ****
>
> Olga Sysoeva,****
>
> Research Associate, PhD****
>
> Psychiatry Department,****
>
> Washington University School of Medicine****
>
> Campus Box 8134****
>
> 660 South Euclid Ave****
>
> Saint Louis, MO 63110-9909****
>
> ** **
>
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