Here is another example of a simple error of omission that could have been caught if the student had read the essay aloud or given it to a friend to read. The word "of" should be between "calculation" and "the." That one small error makes the entire sentence awkward and confusing. If the instructor has to reread the sentence to try to understand its meaning, the flow of the essay is interrupted. If this happens often enough in the essay, it gives an overall bad impression on what otherwise might be a very good paper in terms of research.
A poem says that if you have friendship with a person of bad character you become nameless and formless like the drop of water on a frying pan on the burning oven. The drop of water on a hot frying pan evaporates and disappears. So also a bad friend will spoil your image and you are no longer a good person. Just as good water stinks and becomes black and impure when it mixes with sewage so also a bad friend spoils you and you stink because of your bad qualities. Your fair name is spoilt by your association with a bad friend of the blackest vices.
Never apologize for or otherwise undercut the argument you've made or leave your readers with the sense that "this is just little ol' me talking." Leave your readers with the sense that they've been in the company of someone who knows what he or she is doing. Also, if you promised in the introduction that you were going to cover four points and you covered only two (because you couldn't find enough information or you took too long with the first two or you got tired), don't try to cram those last two points into your final paragraph. The "rush job" will be all too apparent. Instead, revise your introduction or take the time to do justice to these other points.