Clarified use of + and - operators for categorical contrasts w/ group model analysis builder.

Author: jpellman

docs/user/_sources/fsl_ga.txt

@@ -73,6 +73,7 @@ Specifying Contrasts
     * **>** Greater than
     * **+** Positive
     * **-** Negative
+    For categorical contrasts, use the + and - operators.  Using a phenotype file with two groups coded as 1 or 2 will produce an EV for 'group2'.  'group2+' will perform the contrast 'group 2 > group 1' and 'group2-' will perform the contrast 'group 1 > group 2'.  Using these two operators avoids issues of multicollinearity introduced by dummy coding.
 
 #. **f-Tests - [checkboxes]:** Define an f-test by selecting two or more contrasts to include.  When you are done, select the f-tests that you wish to run.  
 

docs/user/fsl_ga.html

@@ -102,11 +102,12 @@ <h3>Specifying Contrasts<a class="headerlink" href="#specifying-contrasts" title
 <ol class="arabic">
 <li><dl class="first docutils">
 <dt><strong>Contrasts - [checkboxes]:</strong> Specify your contrasts in this box. When the model builder builds the design matrix, it will process the categorical variables appropriately and provide the names of the different levels available as contrast labels, listed under &#8216;Available Contrasts&#8217;. When you are done specifying contrasts check the contrasts you wish to run.  The contrast operators available include:</dt>
-<dd><ul class="first last simple">
+<dd><ul class="first simple">
 <li><strong>&gt;</strong> Greater than</li>
 <li><strong>+</strong> Positive</li>
 <li><strong>-</strong> Negative</li>
 </ul>
+<p class="last">For categorical contrasts, use the + and - operators.  Using a phenotype file with two groups coded as 1 or 2 will produce an EV for &#8216;group2&#8217;.  &#8216;group2+&#8217; will perform the contrast &#8216;group 2 &gt; group 1&#8217; and &#8216;group2-&#8216; will perform the contrast &#8216;group 1 &gt; group 2&#8217;.  Using these two operators avoids issues of multicollinearity introduced by dummy coding.</p>
 </dd>
 </dl>
 </li>