Hey, quick question from a nooby "scripter" here

Is there a way to display actor's evasion parameter?
I tried to modify "Draw Parameter" section in Window_Base by adding:
when 7
parameter_name = $data_system.words.eva
parameter_value = actor.eva
self.contents.draw_text(x + 10, y, 120, 32, parameter_name)
self.contents.draw_text(x + 45, y, 36, 32, parameter_value.to_s, 2)
... but when im trying to make script display it by the draw_actor_parameter command it doesnt work. :/ This only results in the game crash with a message which says that the first line is wrong (the one below "when 7") and this:
undefined method 'eva' for #<RPG::System::Words:0x3335778>
I dont really know how to "define method 'eva'" or where to do it. Could someone help me with this?