Well, it may seem odd to some one but I am very much sure there is no criteria of FPSC to mark Essay paper. Obviously, relevant content / material is the prerequisite but sometimes mood of the examiner is the most important thing, sometimes flowery language may be of prime importance, some times facts and figures may do the trick, sometimes quotations may add color to essay and sometimes simple vocab along with good presentation may be the decisive factor and so on....I agree selection of words is contributory factor in success but at the same time what if words are so naive or difficult that even examiner has to consult dictionary to understand its meaning? Obviously, he/she will not feel impressed...

The problem is that every teacher / examiner may have its own preferences which he/she may consider most important for grading Essay. Once, I was reading the essay written by Mufakhar Adeel (2nd position holder in CSS 2006). I was amazed to notice that there was not a single word which could not be understood by even an intermediate student. He scored 66 in that essay. So my advice is to give your essay a bit flavour of all necessary ingredients like relevancy to the topic, simple words along with a touch of some high-vocab words, facts and figures, examples, quotations, arguments etc to be on safer side as you never know preference / weightage the examiner will be giving to certain aspects while marking essay paper.

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Best end of life is not knowledge but action.
Don't limit yourself to what you think people are expecting!