How is a PhD applicant's potential as a researcher evaluated? I have been approaching PhD applications under these assumptions:


The focus is mostly on the letters of recommendation.
Every admitted applicant has several letters that make a convincing argument that the he would be an extremely successful researcher in the given field.
These letters are only credible if they are from reputable professional researchers who know the applicant well.


But how much evidence is necessary to show that the applicant would be a successful researcher? First, what does the recommendation letter author need to see in the student to make that argument sincerely, and how much interaction is necessary before the author knows the student well enough to make that judgment? Second, what evidence does the admissions committee need to read in order to believe that argument?

(I am studying computer science in the US, if it makes a difference.)
The author is supposed to be able to discern and determine some strengths from
the applicant's personality and his academic procedure, could which
considered as valuable points in her view point, to be known and
succeeded as a graduate student. As an instance, the author might
praise his teaching capabilities, based on his benevolence to help
the other students to understand the academic subjects, in a better
form. As an another example, she might assess him as an assiduous
person, as he had summarized many articles to conflate his essay's
introduction, systematically. More interaction she had with the applicant, more
robust explanation she would present about the person... As a matter
of fact, the author can focus on every positive aspect of the
applicant and try to elaborate the details, reasonably...
Readers do not often expect any official evidence for the asserted
points within the LoR. They would be convinced by the letter's
credibility, if the author has shown the relevance between the level
of the support for the person and the depth of the relationship with
him. Obviously, knowing the author's identity would act as an indispensable factor to authorize the LoR and its content.


Good luck