I'm a little confused about scope of variables, in ruby I wrote a test program:

class Test 
        attr_reader :tester 
        def initialize(data) 
                @tester = data 
        end 

        def getData 
                 tester 
        end
end 

puts Test.new(11).getData

now this works fine, the attr_reader, but my confusion is that since I've define attr_reader :tester then why can't I go tester = data rather then @tester = data, because when retrieving the data in getData I only have to write tester and not @tester