[FieldTrip] ANOVA and tails

[Sysoeva, Olga Vladimirovna]

Thank you, I understand all of this. I just do not understand why I can have more liberal, less conservative criteria (2 tail) in my ANOVA design here in Filedtrip.
I do not have any prior expectation about the result ( for example both increase in value for positive component , such as P300, and decrease in value for negative component (such as N1) can be considered as increase in the amplitude of this components).

From: fieldtrip-bounces at donders.ru.nl [mailto:fieldtrip-bounces at donders.ru.nl] On Behalf Of Gopakumar Venugopalan
Sent: Thursday, September 01, 2011 11:58 AM
To: Email discussion list for the FieldTrip project
Subject: Re: [FieldTrip] ANOVA and tails

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<mailto: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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