In my opinion speaking is very hard to assess since it is a productive skill you as a tester will have to be very receptive in order to see if what the learner has produce is actually what you need to get from him. I believe that, in order to make a valid test you will have to define the treats you wish to evaluate. As Brown suggest (pronunciation, fluency, vocabulary use, grammar, comprehensibility, etc.) are some traits you can use to get part of the score, of course, the more you specified your goal from the speaking test the more “fair” (if could say) your grade would be, and to be specific you can create new traits.
