addEventListener not working in IE8

Try:

if (_checkbox.addEventListener) {
    _checkbox.addEventListener("click", setCheckedValues, false);
}
else {
    _checkbox.attachEvent("onclick", setCheckedValues);
}

Update:: For Internet Explorer versions prior to IE9, attachEvent method should be used to register the specified listener to the EventTarget it is called on, for others addEventListener should be used.

You have to use attachEvent in IE versions prior to IE9. Detect whether addEventListener is defined and use attachEvent if it isn't:

if(_checkbox.addEventListener)
    _checkbox.addEventListener("click",setCheckedValues,false);
else
    _checkbox.attachEvent("onclick",setCheckedValues);
//                         ^^ -- onclick, not click

Note that IE11 will remove attachEvent.

See also:

• MDN: element.addEventListener: Legacy Internet Explorer and attachEvent
• MSDN: attachEvent method