[FieldTrip] Antw: Re: ANOVA and tails

Fri Sep 2 13:36:20 CEST 2011

Thank you Gregor, I said the converse of everything I wanted to say. I
appreciate the clarification. I know one thing never type or speak the day
you have been sleep deprived. In my case I end up saying the opposite of
everything I intended to say (tests of significances included!).

Warm regards
gopa

On Thu, Sep 1, 2011 at 11:45 PM, Gregor Volberg <
Gregor.Volberg at psychologie.uni-regensburg.de> wrote:

>  Dear Olga and Gopa,
>
> I would like to comment on this discussion because I fear that there could
> be some misunderstandings.
>
> Unlike t, the F statistic is not sensitive to the direction of a
> difference. The  t value gets positive if means of group A > group B, and
> negative if B > A. Therefore, the hypothesis that A >< B can be tested on
> both tails of the t distribution. On contrast, the F value gets positive if
> A > B as well as if B > A. Small values of F, on the left tail of the
> distribition, indicate that there is no difference between means of A and B.
> This is why a left tail or a two-tailed test does not make sense with an
> F-Test.
>
> So Olga, if you have a two-group design, and if you want to apply a left-
> or two-tailed test, then use the t statistic. Beware that the two-sided test
> is usually considered more (not less) conservative that the one-sided test.
> This is because the critical value of t increases for two-sided tests. It is
> the size if the rejection region for H0 that needs to be halved, not the
> one-sided critical value of your test statistic. For example, a result of
> t(20)= 1.9 is significant in a one-tail right-sided test where it exceeds
> the critical value of 1.72 (the 0.95 quantile of a t distribution with 20
> df). But it is not significant in a two sided test where the critical values
> are 2.09 (0.975 quantile) and -2.09 (0.025 quantile).
>
> Best regards,
>
> Gregor
>
>
>
>
> --
> Dr. rer. nat. Gregor Volberg <gregor.volberg at psychologie.uni-regensburg.de>
> ( mailto:gregor.volberg at psychologie.uni-regensburg.de )
> University of Regensburg
> Institute for Experimental Psychology
> 93040 Regensburg, Germany
> Tel: +49 941 943 3862
> Fax: +49 941 943 3233
> http://www.psychologie.uni-regensburg.de/Greenlee/team/volberg/volberg.html
>
>
>
> >>> Gopakumar Venugopalan <venug001 at crimson.ua.edu> 9/1/2011 6:58 PM >>>
>
> 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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