X3</em> given <em>X1</em>. Cross-validation is performed either on the rows of <em>X1</em> or the columns of <em>X1</em>.</p>
<pre><code>   ______N 
  |       |
  |       |
  |  X3   |
  |       |
 K|_______|
             
             
   ______N       ________J 
  |       |     |         |
  |       |     |         |
  |  X1   |     |   X2    |
  |       |     |         |
 I|_______|    I|_________|</code></pre>
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<a hre