bb wrote:
Thanks Ian! In my book, you are the ultimate authority on GMAC algorithms and points.
Is this wrong? I have seen a few discussions about the complex calculation they use but I think the back of napkin answer I always give - 3 percentile points per unanswered question. This number supposedly varies depending on how low high the score is, but that’s the fast dirty and sometimes accurate answer or is it really wrong?
A long time ago, in a couple of places (an interview with Lawrence Rudner, their chief psychometrician, and in an online article, if I recall correctly), official GMAC reps explained that the penalty for not finishing was "proportional". This was back when the Quant section had 37 questions, so if you didn't answer three questions, say, your 'score' was multiplied by 34/37. The problem though is that they never specified just what 'score' was being multiplied by that proportion -- the scaled score, the percentile, or the raw internal score (which you never see, and which is between about -3 and +3) that the algorithm uses? They do give one numerical example of the penalty in the
OG, in a "Myth vs Fact" bubble (I have OG2018 in front of me, and it's on pg 13). There it says "failing to answer five Verbal questions.... could reduce your score from the 91st percentile to the 77th percentile". When they first published those numbers, a V41 corresponded to a 91st percentile score, and a V36 to a 77th percentile score (the percentiles have changed since then), and that made me think they were applying the 36/41 penalty in this case directly to the scaled score of V41 -- the numbers just worked out perfectly. But it's impossible to be sure of a conclusion drawn from just one data point, and I've seen score adjustments since then that have led me to think the penalty probably isn't applied to scaled scores.
Long story short, I'm not sure of the exact answer, and no one else will be either (outside of the GMAC offices). I also haven't seen any official comment about the penalty since the test was shortened (from 37 Quant questions to 31 and similarly for Verbal) -- it's certainly possible they modified the calculation of the penalty when they abbreviated the test. I'd guess that your estimate, of three percentile points per unfinished question, is going to be quite a good estimate for test takers who are in the average to above-average range. Because the penalty is proportional, it is less severe for low-level test takers (if you're reducing a score by 10%, the reduction is smaller for a Q20 test taker than for a Q50 one). So your estimate might not work well for test takers at extremes of the scoring range, or in extreme situations where, say, a dozen questions are left unanswered -- but I don't know for sure, it's also possible that it will be perfect in those cases! No one (as far as I'm aware) knows the exact answer, and it's unfortunately impossible to test using GMATPrep tests or the official online tests (because there's no way to know, after, say, 25 questions, what score the test thinks you have -- some people might think they can test the penalty by answering every Quant question perfectly, to get a Q51, but the Quant scoring scale goes above Q51 internally so that doesn't work).
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