tableObject.tFootコレクション

$tableElementReference.tFoot;

$tableElementReference.tFoot; // 表（table要素）のフッタ（tfoot要素）への参照

$tableElementReference.tFoot.rows[0].cells[0]; // フッタ内の１列目のセルへの参照

<script type="text/javascript">
window.onload = function(){

    var $sampleTable = document.getElementById( "sample" );
    var $sampleColgroup = $sampleTable.getElementsByTagName( "colgroup" )
    var $sampleTHead = $sampleTable.tHead;
    var $sampleTHeadCells = $sampleTHead.rows[0].cells;
    var $sampleTBodies = $sampleTable.tBodies;
    var $sampleTFoot = $sampleTable.tFoot;
    var $sampleTFootCells1 = $sampleTFoot.rows[0].cells;
    var $sampleTFootCells2 = $sampleTFoot.rows[1].cells;
    var $sumBySubject = [];

    for ( var $tbodyIndex = 0; $tbodyIndex < $sampleTBodies.length; $tbodyIndex++ ) {
        var $sampleTBodyRows = $sampleTBodies[$tbodyIndex].rows;
        for ( var $rowIndex = 0; $rowIndex < $sampleTBodyRows.length; $rowIndex++ ) {
            var $sampleTBodyCells = $sampleTBodyRows[$rowIndex].cells;
            for ( var $cellIndex = 1; $cellIndex < $sampleTBodyCells.length; $cellIndex++ ) {
                $sumBySubject[$cellIndex] = 0;
            }
        }
    }
    for ( var $tbodyIndex = 0; $tbodyIndex < $sampleTBodies.length; $tbodyIndex++ ) {
        var $sampleTBodyRows = $sampleTBodies[$tbodyIndex].rows;
        for ( var $rowIndex = 0; $rowIndex < $sampleTBodyRows.length; $rowIndex++ ) {
            var $sampleTBodyCells = $sampleTBodyRows[$rowIndex].cells;
            for ( var $cellIndex = 1; $cellIndex < $sampleTBodyCells.length; $cellIndex++ ) {
                if( $cellIndex != 0 ){
                    $sumBySubject[$cellIndex] += parseInt( $sampleTBodyCells[$cellIndex].innerHTML );
                }
            }
        }
    }

    for ( var $cellIndex = 1; $cellIndex < $sampleTFootCells1.length; $cellIndex++ ) {
        $sampleTFootCells1[$cellIndex].innerHTML = $sumBySubject[$cellIndex];
    }
    for ( var $cellIndex = 1; $cellIndex < $sampleTFootCells2.length; $cellIndex++ ) {
        $sampleTFootCells2[$cellIndex].innerHTML = Math.round( $sumBySubject[$cellIndex] / ( $sampleTable.rows.length - $sampleTHead.rows.length - $sampleTFoot.rows.length ) );
    }

}
</script>