absolute value or squaring will compute absolute error and mean square error. Note that the absolute value and square operation should <em>not</em> be done within the Stan program because neither is a linear function and the result of averaging squares is not the same as squaring an average in general.</p>
<p>Because the test set size is chosen for convenience in cross-validation, results should be presented on a per-item scale, such as average absolute error or root mean square error, not on the scale of e