Filtering out the Free-Riders: the 15/40 Rule

My last post on this topic was about encouraging everyone to work together on quizzes by making them all take-home and not scaling anything.  The benefit is that it fosters community and valuable people-skills, but it has a drawback:  that a student with little grasp of the material may make it through the class on the efforts of others, copying the quizzes of students who have worked hard to learn the material.

free rider
I should have done this long ago, but starting a couple of years ago I instituted the “15/40 Rule.”

Here’s how it goes:
If a student’s final exam is at least 15 points above their quiz average then the quiz average is automatically raised to 15 below the final exam grade.  This allows a student who took longer to master the material or missed a quiz due to life circumstances to still do reasonably well.

If a student’s final exam is at least 40 points below their quiz average, then the quiz average is automatically lowered to 40 above the final exam grade.  This filters out the occasional free-rider who  copied other people’s work but couldn’t get into the lower 40s on their final exam, and ensures that nobody who doesn’t at least pass the final exam gets above a C+.

Typically about 10% student are bit by the “40” part of the rule.  I think once someone was helped by the “15,” but it’s extremely rare.

Overall I’m quite happy with the way it works.  I’ve thought about tightening it to a 10/30 or even 10/25 Rule or something like that, but worry that the Powers That Be may get on my case if too many people fail the class.  Also, if people start to feel like their quiz grades won’t matter, they may try less hard on the quizzes and thus learn the material less well, making failing the class a self-fulfilling prophecy!

Does anyone else do something like this?  Any thoughts on whether the rule should be tightened or not are welcome.  Currently the final exam represents roughly half of the total grade, so the maximum grade is more-or-less the final exam + 20.

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