QDEC creates a sentence that describes each contrast. Is that sentence not clear? I think the only other docs we have for QDEC are the tutorial.
doug

On 08/19/2013 12:19 PM, ye tian wrote:

Dear Doug,

If I only have two groups, does the statistical test reduce to a two-tail t-test?

Sorry, I meant to ask what the "default" contrast is, meaning what the contrast is when I specify no nuisance factor?

Is there an official document where the QDEC setup is gone over systematically?

Thank you very much!

Sincerely,
Ye

On Mon, Aug 19, 2013 at 11:04 AM, Douglas N Greve <greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu>> wrote:


    -log10(p) which p is the pvalue for the statistical test. The
    color is signed by the test.



    On 08/19/2013 11:19 AM, ye tian wrote:

        Dear Doug,

        Would you please shine some light on what quantities are being
        plotted in those statistical maps of QDEC? Some linear
        combination of the columns of the design matrix?

        Also, what is the "default" design matrix?

        Thank you very much!

        Sincerely,
        Ye


        On Tue, Aug 13, 2013 at 11:45 PM, Douglas Greve
        <greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.edu>

        <mailto:greve@nmr.mgh.harvard.edu

        <mailto:greve@nmr.mgh.harvard.edu>>> wrote:


            1 and 3 are the same model. Calling a variable a nuisance
        factor
            is just a description. The model is actually
            y  = age*meditation*b1 + meditation*b2 + err = y_hat + err

            doug


            On 8/13/13 6:12 PM, ye tian wrote:

                Dear Freesurfers,

                I am studying cortical thickness as a function of age
                (continuous) and meditation practice (categorical,
            level1=Normal,
                level2=TM). I have three separate cases of qdec,
            regression1,
                regression2 and regression3.

                Regression1: Discrete (Fixed Factor) = meditation;
            Continuous
                (Covariate) = age
                Regression2: Discrete (Fixed Factor) = meditation;
                Regression3: Discrete (Fixed Factor) = meditation;
            Nuisance
                Factor = age

                For the question "Does the average thickness differ
            between
                Normal and TM?", Regression1 and Regression3 have
            identical
                result, which is different from Regression2, i.e., (1
            = 3 !=2 ).
                May I infer from this result the following about qdec:

                1) Regression1 and Regression3 both fit cortical
            thickness,y, to
                the model
                    y  = b0 + age*b1 + meditation*b2 + err = y_hat + err
                    Regression2 fits:
                    y = b0 + b2*meditation + err = y_hat + err

                2)  Some function of y_hat goes into the "average
            thickness",
                hence the difference between 2 and the others.

                However, what role does "Nuisance Factor" play in this
            case?

                Thank you very much!

                Sincerely,
                Ye





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