Standardized tests are a useful way to determine a student’s ability, relative to that of his or her peers, because the tests are designed such that there will always be an equal ratio of high scores to medium scores to low scores.
This allows us to assign each score a percentile — a figure that represents where a score sits on a distribution curve.
As you can see, though, that's clearly not how the essay value is calculated, although these nominal amounts are within the observed range for most essay scores.
The precise amount will be calculated based on the observed distribution of scores for that essay.
Such an answer is unsatisfying, however, so I set out to derive a more specific answer by inferring the unrounded contribution of particular essay scores based on the score scales released with publicly available tests.
[Update (12/6/2013): I've added more scales to my analysis (nearly doubling the data set) and refined my estimation algorithm so that it gives a more precise result for many scales.The other steps average about 15-16 scaled-score points for a one-point increase in the essay score. I started by assuming that ETS would follow normal psychometric practice in creating a composite scale.That means that the composite writing score should be calculated in one of two ways: either by adding weighted raw scores to produce a composite raw score, which is then translated to a scaled score, or by assigning separate scales to the multiple-choice portion and the essay portion and adding the two to produce the final composite scale.Notice that there's more variability in the extreme scores on both ends, but especially at the high end, than there is for the middle scores.The percentages of 11s and 12s seem to vary significantly more from test to test than any other score point.Indeed, for some tests, there's essentially no practical difference, after rounding, between a 0 or a 2 on the essay.The next smallest payoff is the step between 4 and 5 (only 11.8 points).Based on the pattern of numbers in the scale tables, I strongly suspected that the second method was the one used, but I checked them both out to be sure.I could not find any set of numbers that could explain the observed score tables under the assumption that weighted raw scores were summed, but I found solutions for every score table I tried under the second method.Under this method, we can think of a composite scaled score as the sum of a multiple-choice scaled score (NB: not the same as the multiple-choice subscore reported on the test report) and an essay scaled score.In other words, $S_ = S_m S_e 200$, where $S_m$ is the scaled-score contribution for a multiple-choice raw score of $m$, and $S_e$ is the scaled-score contribution for an essay score of $e$.