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You're gonna want to use AJAX and submit the form, then you can access the returned data without ever refreshing the page.

Basically:

HTML

<select name="combo" id="combo_1">
 <option value="2">Submative</option>
 <option value="1">formative</option>
</select>

JavaScript

$('#combo_1').change(function() {
 $.post('calcScript.php', $(this).serialize(), function(data) {
 alert(data);
 });
});

in PHP, you can access your combo data via $_POST['combo'].