2 yak or not 2 yak...

I found a fabulous blog today by exzuberant who focuses on math in the classroom.  Many posts caught my eye and I'll be spending a lot of time going back and reading the ones I didn't have time for but I wanted to talk about the original one that jumped out at me.

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A short excerpt: 
Would you like 2 yaks or 3 yaks with that test?
I took a (small) risk last week and tried something different for our Year 8 Algebra test. I'm calling it the "2-Yak/3-Yak test". The idea in a nutshell: students choose the level of difficulty of the test.

Each section of the test provides questions grouped into level of difficulty indicated by the number of yaks. Students were required to do the 2-Yak column, and then for each section choose between the 1-Yak or the 3-Yak column. If students selected the 2-Yak/3-Yak combo, they would automatically get the marks for the 1-Yak questions. I suggested to students that if they wanted to do the 3-Yak questions but thought they might be too hard, to just do the 1-Yak/2-Yak, move on to the next section - and then at the end, if they had more time, go back and try some 3-Yak questions.

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I thought this was such an interesting concept!  Every student is required to attempt column 2 and then based on their skill level, attempt 1 or 3.

My only concern would be that some of my stronger students would attempt column 1 because they know that they'll get it right whereas column 3 might cause them to lose marks.

Exzuberant answers this question as follows:

It turned out to be quite easy to allocate automatic 1-Yak marks to students who didn't do that column. In most cases I gave them full marks - although if I saw the student didn't really understand the concept I docked a mark. But it was very clear there was no need for these students to do the 1-Yak questions

I ended up giving all students two scores: a Baseline score (1-Yak+2-Yak or 2x 2-Yak) and an Extension Score based on the number of correct 3-Yak questions. In my markbook, I recorded these in two columns - treating them as two different tests. For my final grading, I will weight them so students doing the 3-Yak question get the equivalent of 0.5 marks extra per 3-Yak question.

When you're focusing on Standards based grading, this system is a lot easier to work with.  I'm not exactly sure how I can transfer these marks to my own grading system.

Question to You:   If you were to try a test like this, how would you determine a final mark?