Taking suggestions for grading rubric for multiple answers test We're being encouraged to use multiple answer tests at my school.  In case you're not familiar with the concept, each question may have multiple correct answers; students are encouraged to mark all valid answers.

There's at least one correct each for each question.  Each question has either 4 or 5 answers to select from.  It is possible that all answers are correct.

I would like to grade this so that, even when there are multiple correct answers, if the student only selected one answer and it is correct, they receive partial credit.

What I'm not sure about is how to grade problems in which:


all answers have been selected, but not all answers are correct.
some of the selected answers are correct and others are incorrect.


We've been given no guidelines beyond what I've mentioned above.  When asked about how we should grade these tests, we've been told no more than to try to award partial credit if at least one correct answer has been selected.

Does anyone out there have experience grading tests like this?  If so, how might you design a grading rubric.
One simple way to design a grading rubric for this situation is to consider the question as though it were a set of true-false questions.  So if there are 5 answer and the question is worth five points, then there is 1 point for marking each true, and 1 for not marking each false answer.

Thus, for example, consider the following question: 


  Which of these are mammals? (5 points)
  
  
  Cat
  Turtle
  Lizard
  Fish
  Dog
  


If the student marked "Cat, Turtle", then they would receive 3 points: they lose 1 point for marking "Turtle" and 1 point for failing to mark "Dog."