Why Doesn't My Code Work?

I am trying to make it so if a user checks a radio button within a group of 2 (Choice1 & Choice2) the formula within the field "Total" changes.

This is the javascript within the field "Total"

//////////////////////////////////////////////////////////////////////////////

if(document.Group1('Choice1').checked)

{

event.value = (this.getField("WSAtotal").value + this.getField("SAandAdd").value);

}

else if(document.Group1(Choice2').checked)

{

event.value = (this.getField("NSAtotal").value + this.getField("SAandAdd").value);

}

//////////////////////////////////////////////////////////////////////////////

P.S. I don't know anything about javascript so I have just been Frankensteining this code from around the web.