Hi Dennis:
Your exercise is wonderful - meaningful and valuable. Why is it extra credit? If it's important, why not incorporate it into the regular assignment grades.
EC implies "special points for something special". Unless it's offered to all, graded the same, and doesn't add piles of extra points, I'd avoid EC if possible. Call it bonus points; then all students can have a stab at it and you can grade accordingly. EC given in substitution for regular classwork just seems unfair, and diminishes the hard work your good students have put in to ear their true grade.
I don't… >>>