well my teacher mentioned that "insight" is THE marking criteria for all english questions. so its a matter of how much "insight"you've shown to the examiners instead of the quantity of your essay.
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the following is my theory, base on the assumptions:
1.) you know what you are gona write and you can write 20 pages if you have enough time
2.) you DONT have enough time
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as they adopt "positive marking" approach, i.e. rewarding marks instead of deducting marks, then the more quantity you put down, the more chance you can get marks, as long as you can manage to make sure that what ever u've put down is at least LIKELY to get marks
As a result, u should be lucky enough to "accumulate" a b6
HOWEVER, as there is an "impression marking" element i.e. overall structure and fluency of the essay, as well as the way you present your arguement, then there are risks involved in emphasizing quantity:
1.) as you write too much, you start to lose subtlety because you may start to repeat phrases .ect
2.) your essay is no longer "concise"
3.) illegible handwritting <-my problem, which means if they cant read it, they are not gona reward u marks
consequently, when they have to decide on "what should i give this guy, 17?18?19?" then they will go for "17" instead of "19" due to "impression marking" and they have the legitiment reasons to do so