function howMany(thisID) {
	lastCheckbox = document.getElementById(thisID);
	elements = document.getElementsByTagName('input');
	var length = elements.length;
	var total_checked = 0;
	for(var i=0; i<length; i++){
		current = elements[i];
		
		if(current.type == 'checkbox' && current.checked) {
			total_checked++;
			if(total_checked > 3) {
				alert("There are already 3 items in your comparison list.\n\rPlease remove an item before attempting to add another.");
				lastCheckbox.checked = false;
			}
		}
	}
}