Letter of Recommendation for Graduate Applicants
The importance of the letter of recommendation in the admissions process:

American universities require data and information in order to process and evaluate a request for admission: the application form, results of standardized tests, academic records, a personal statement, and proof of financial solvency.

In addition to this information, letters of recommendation, generally three in number, must accompany the request for admission.
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Model Letter
Strategies for obtaining Letter of Recommendation
What if you don't have a Recommender.
These letters are of great importance for the candidate by virtue of their influence on the final evaluation and decision of the university. The information furnished by a good letter of recommendation provides relevant support material and assists the institution in building a clear profile of the candidate in terms of previous activity, academic potential, personal attributes and future professional application of knowledge acquired through study in the USA. In some cases, a letter of recommendation may be the deciding factor in a candidate's admission.

Whom to ask for a letter of recommendation?
Carefully choosing the people who will write about him or her is in the candidate's best interest. It is preferable that the letters be written by professors, although one of two may be from among those who employ or supervise the candidate. It is important the people chosen be knowledgeable about the candidate and also qualified to evaluate his/her performance and to provide personal, specific and meaningful information about the candidate.