Per the AAMC:
Understanding Your Scores
How is the MCAT scored?
Four separate scores are derived from the MCAT, one for each section and the total score. Each score that you achieve on the multiple-choice sections (Physical Sciences, Verbal Reasoning, and Biological Sciences) is based on the number of questions you answer correctly. This raw score is a reflection of your correct answers only. This means that a wrong answer will be scored exactly the same as an unanswered question; there is no additional penalty for wrong answers. Therefore, even if you are unsure of the correct answer to a question, you should make your best guess.
Your raw score on the Writing Sample is determined by adding the scores you receive on each of the two responses you write. Because two different readers rate each response, your total raw Writing score is the sum of the four scores: two for the first response and two for the second. From the raw scores we calculate scale scores (see below for why we do this), and these scale scores appear on your final score report. The scores for the multiple-choice sections—Verbal Reasoning, Physical Sciences, and Biological Sciences—will be reported on a scale ranging from 1 (lowest) to 15 (highest).
The raw score you receive on each section is converted to a score on this 15-point scale. For example, if your raw score on one of the sections is between 40 and 43, your converted score might be 11. Scores ranging from 44 to 46 might have a converted score of 12, and so forth.
Your raw score on the Writing sample will be converted to an alphabetic scale ranging from J (lowest) to T (highest). Each letter represents the sum of two scores on the two Writing Sample items. The sum can result from different combinations of individual scores. (Individual scores are assigned along a 6-point scale.) For example, a student whose scores are 4 and 5 on the first item and 4 and 4 on the second—a raw score of 17—would receive the same alphabetic score point as student who scored a 3 and 3 on the first items and a 5 and 6 on the second.
In addition to scores for the individual sections, a total score will also be reported. This total score will consist of a combined multiple-choice score conjoined with the Writing Sample score, e.g., 42T.
Why are raw scores converted to scaled scores?
The conversion of raw scores to scaled scores compensates for small variations in difficulty between sets of questions. The exact conversion of raw to scaled scores is not constant; because different sets of questions are used on different exams. Conversion of your raw scores to the 15-point scale also helps minimize variability in test scores due to factors unrelated to your skill or knowledge. Variability in scores may be due to such external factors as state of health or degree of familiarity with standardized testing. The 15-point scale tends to provide a more stable and accurate assessment of a student's abilities. Two students of equal ability would be expected to get the same scaled score, even though there might be a slight difference between the raw scores each student obtained on the test.
Scaled scores on the MCAT can be interpreted as percentile rank ranges based on the performance of all students taking the test during a given year.
Is the exam graded on a curve?
Examinees often ask if earning a high score or higher percentile is easier or harder at different times of the testing year. They ask whether they have a better chance of earning a higher score in April or in August, for example. The question is based on an assumption that the exam is scored on a curve, and that a final score is dependent on how an individual performed in comparison to other examinees from the same test day or same time of year.
While there may be small differences in the MCAT exam you took compared to another examinee, the scoring process accounts for these differences so that an 8 earned on physical sciences on one exam means the same thing as an 8 earned on any other exam. The percentile provided on your score report simply indicates what percentage of examinees from the previous testing year scored the same as you did on the MCAT exam.
How you score on the MCAT exam, therefore, is not reflective of the particular exam you took—including the time of day, the test date, or the time of year—since any difference in difficulty level is accounted for when calculating your scale scores (see above for information about scaling).
http://www.aamc.org/students/mcat/preparing/understandingscores.htm
