function getAverage ( $iStat ) {
    for ( $i=1; $i<30; $i++ ) {
      $sql = sprintf( "SELECT avg( value ) as AVG FROM data WHERE statistic_id = '%d' and day = CURdate()-INTERVAL %d day", $iStat, $i );
      $res = mysql_query( $sql );
    $obj = mysql_fetch_object( $res );
    if ( ! $obj )
      return false;
    return $obj->AVG;
  }