<script type="text/javascript">
$.validator.methods.checkboxGroup = function(value,element,param)
{
  return (
    $("input[name='length']").is(':checked') ||
    $("input[name='density']").is(':checked') ||
    $("input[name='packaging']").is(':checked') ||
    $("input[name='loose']").is(':checked')
  );
}
 
$('form').validate({
  rules: {
    length: "checkboxGroup",
    density: "checkboxGroup",
    packaging: "checkboxGroup",
    loose: "checkboxGroup",
    //etc
  },
  errorPlacement: function(err, el) {
    var name = $(el).attr('name');
    if (name == 'length' || name=='density' || name=='packaging' || name=='loose')
    {
      err.insertBefore(el);
    }
    else
    {
      err.insertAfter(el);
    }
  }
});
</script>

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