define dissertations

What does the word ‘debate’ imply? A discussion involving different points of view or sets of ideas. A dissertation will therefore not only examine a subject but will review different points of view about that subject.
There are some obvious differences: an essay is relatively short – usually 1500 to 2500 words – and you are told clearly what to do by someone else. For example: Describe and evaluate major theories of globalisation.

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http://llmphd.uottawa.ca/en/what-difference-between-masters-thesis-and-research-paper