Thursday, July 16, 2009

USMLE Scores and Percentiles (simple guide)

2 digit scores are standardized so that the minimum passing score is a 75 and the maximum score is a 100. This approximately correlates to each digit between 75 and 100 being 4 percentiles of those who passed. The 2 digit score is something requested/required by state medical boards for all licensing exams so they can quickly evaluate the score without figuring out each exam's scales.

Scores used to be calculated after a group of students took them and you were given a 2 digit score, a 3 digit score, and a percentile. They used to do it where a large number of students took the exam, then they were all graded and statistics were calculated based upon all the students taking it then. The percentile was where you stood among the other students who took the exam in the block from which your exam was taken and scored. Therefore, each year, the median score would get a 50th percentile, although one year this could be a 205 and the next a 210. So a 240 in 1990 might be a lower percentile than a 235 in 1991. The problem is that a 240 is doing better, but 1991 may have been a "dumber" year so you could have a higher percentile with a lower score.

Now with computerized exams, they do not score your exam relative to the others taking it when you do... it is referenced to historical performance of the test items at which time a "difficulty level" for each item was assigned. This way, a 240 is a 240 no matter when you take it. Also, you do not have to wait for thousands of students to take the exam, do all your statistics, and then calculate the scores... you can calculate the score immediately. The implication of this, however, is that it is impossible to calculate a percentile since you are being scored against an inertia-laden grading curve. Every student could get a 240 now, and every student can fail.

There are many implications of this grading policy, and understanding it (really understanding it, not just saying that 240 is good and my score report says the mean was x with a sd of y) will serve you well.

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