Ursula Jones, of Gwinnett County, Georgia, says that she just gave a full-length old AP exam to her class for review. She asks, how do you convert the scores to a 5, 4, 3, 2, 1 AP equivalent, and then how might she convert that to a standard 100 point school scale.
First of all, know that the score conversions change slightly each year, depending on how difficult the free response section is. ETS statisticians look at the global average scores on the multiple choice section, especially on the subset of questions that appeared on previous years' exams. From that, they can estimate whether the free response was "easy" or "hard," and adjust the score conversions accordingly so that a 5 means essentially the same thing from year to year.
The most recent released score conversions come from the 2009 exam. Here's a link to the physics B conversion chart. After weighting the multiple choice and free response sections equally to 1 point per minute, it gives 62%* for a 5, 47%** for a 4. That's typical, and there's nothing wrong with using that scale for all your in-class tests.
* 112 out of 180
** 85 out of 180
Ursula's second question is more complicated -- converting that to a 100% standard school scale. Students don't like to hear "you got a 5, nice job." They see their score as 62%, and tell everyone they failed. It's useful to be able to write "90% -- A-" on such a student's paper.
Since the AP grades of 5, 4, 3, 2, and 1 represent the equivalent of an A, B, C, D, F in a college course, I make sure they convert just this way. Sure, earlier in the year for regular-season in-class tests, I used test corrections to ensure that a 3 could convert to a B or B- rather than a C, and that 2s could convert to Cs, not Ds. But we're in May, and the real AP exam is around the corner. For a trimester exam, or for a final mock exam, I make the conversion directly.
To convert to the school's 100 point scale, I call the lowest possible 5 a 90, the lowest 4 an 80, and the lowest 3 a 70. Then I just prorate: on the 180 point scale in the linked physics B conversion chart, there are about 27 points in both the "4" range and the "3" range. So every every 2.7 raw points on the AP exam raises the student's school-scaled score by 1 point. Point is, something right in the middle of the 3 range earns a 75, or a solid C.
I'm sure someone reading this blog could write a nice spreadsheet to make these conversions automatic. Please, someone, do it, and post the file somewhere. I'm decent at excel, but I've always just divided out the values by handheld calculator. Please feel free to revoke my nerd credentials.
Hope that helps... I'm sure I could be convinced to write the precise lookup table for each score between 0 and 180. Let me know if you need that. :-)