[VoxBo] g design questions

[Dongbo Hu]

On Thu, 14 Sep 2006, Charan Ranganath wrote:

>_Hi, I've realized that the g-design widget has evolved quickly and that
>_there are now a number of parameters now available that I don't fully
>_understand. Consequently, I've been going consistently with the default
>_settings, but I want to see what they mean and when to use each setting.
>_
>_I'll start with 3 questions:
>_(1) When adding a diagonal set, you have a choice between "do not scale"
>_and "scale by trial count". Do not scale is checked by default. When
>_would you want to scale an element of a diagonal set by a trial count,
>_and how exactly does it work?
>_

This option (and the next one) are added to make "add diagonal set"
operation more consistent with "add contrast", because diagonal set is
basically the covariates with contrast matrix of "1 0 0 ... " (depends on
how many unique keys are available in condition function).

What "scale by trial count" option does is exactly saem as the one in "add
contrast" interface. It counts the number of elements that equals 1, then
divide the 1's by this counter number. For example, If one of the
covariates in diagonal set is:

1
0
0
1
0
1

After "scale by trial count", it will become:

0.333
0
0
0.333
0
0.333

(Because the number of 1's in the covariate is 3, so each element of 1 is
divided by 3.)

Before this option was introduced in diagonal set, diagonal set covariates
were not scaled by trial, so it is turned off by default.

>_(2) The next choices when adding a diagonal set are "leave offset" or
>_"center and normalize," the latter of which is set by default. What does
>_this mean, and what are the implications of using one or the other?
>_

Again, this option is added to make it consistent with "add contrast"
operation. What it does is actually mean-center the covariates. Since the
older version mean-centers diagonal set silently, it is turned on by
default.

>_(3) At the bottom right of the g-design widget window, there is another
>_check box that says, "mean center all but intercept before saving". I
>_believe this is checked by default. What are the implications of doing
>_this, and when would you NOT want to do it.
>_

This option is added because we noticed that most of the users will want
to mean center all covariates except intercept before finishing the G
design. The earlier version didn't do it automatically. This may cause
a compatibility problem, but mean-center operation only shifts coavariates
up or down, so it should be ok in most cases.

>_Finally, just to verify backward compatibility, do the default settings
>_correspond to what was set by default in earlier versions of VoxBo?
>_
>_Thanks in advance for your wisdom, CR
>_
>_--
>_Charan Ranganath, Ph.D.