How to Sort by "Reviewed" column on "My reviews" page

After looking into the conference.module file I found this code and added this line to it:
array('data' => t('Reviewed' ), 'field' => 'Reviewed' , 'sort' => 'asc'), but is causing this error:

user warning: Unknown column 'Reviewed' in 'order clause' query: SELECT * FROM conference c LEFT JOIN node n ON c.pnid = n.nid WHERE c.ruid = 21 ORDER BY Reviewed ASC LIMIT 0, 10 in /var/www/html/conference_search/sites/all/modules/conference/conference.module on line 1403.

Obviously its looking for the "Reviewed" column but don't know where to add it.

Here is the CODE:

function conference_reviews() {
  global $user; $uid = $user->uid;
  if ( ! $ctype_review = variable_get('conference_ctype_review', 0)) {
    // don't tell the reviewer about the missing setup problem...
    drupal_set_message( "There are no reviews assigned to you." );
    return;
  }
  $content = t('The following table contains all papers assigned to you:');

  // table header
  $header = array(
    array('data' => t('Title' ), 'field' => 'title' ),
    array('data' => t('Submitted' ), 'field' => 'created' , 'sort' => 'asc'),
    array('data' => t('Download' )),
    array('data' => t('Reviewed' ), 'field' => 'Reviewed' , 'sort' => 'asc'),
    array('data' => t('Operations' ))
  );
  $rows= array();

  // get papers
  $sql = "SELECT * FROM {conference} c LEFT JOIN {node} n ON c.pnid = n.nid WHERE c.ruid = %d";
  $sql .= tablesort_sql($header);
  if ( $result = pager_query($sql, 10, 9, NULL, $uid))
  while ( $item = db_fetch_object( $result )) {
    // choose some operations
    if (! $item->rnid)     
      $op = l( t("create review"), 'node/add/'. str_replace('_', '-', $ctype_review), 
              array ( 'attributes' => array('title' => 'Create A Review'), 
                      'query' => 'pnid='. $item->nid.'&pvid='.$item->vid)
              );
    else 
      $op = l( t("view/edit review"), 'node/'. $item->rnid);

Thank you for your help.

Ceci