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How to write a letter of interest for grad school
Set your intentions.
You should know what you want to achieve by attending graduate school before you start to apply. This statement is part of your application but it is also an opportunity for you to do some serious self-evaluation about your goals and plans.
While you may not know your exact dissertation topic at this point, you should have a good idea of where you'd like to go in your studies. Evaluate your goals based on what you would like to study, and think about whether your particular focus will contribute to the field you're entering.