According to this MIT statement of purpose guideline, the statement of purpose seems to be an important part of a PhD application. However, I have also heard that, in the case of MIT, the statement of purpose is only reviewed after the application has been tacitly accepted.
So, how crucial is this part of the application, and what part does it play in the process of graduate admissions at top schools in the US?
You have to specify which discipline, but for the social sciences the statement of purpose is the most important item in your portfolio.
We also look at grades, GREs, and letters but we scrutinize the statement the most. A good statement can resuscitate a candidate with middling grades, a bad statement can condemn an otherwise good student.
This is an extremely general question, since it doesn't refer to the discipline. Since another answer comments on the social sciences, I'll speak up for physical sciences. (I'm in chemistry.)
We look at every facet of the portfolio. To do otherwise is to ignore useful information on the applicant. Let's be honest, we don't have much data. We get grades, GRE general scores, GRE subject scores, and recommendation letters. Plus the statement.
I mentioned in my answer to another question that GRE scores will sometimes serve as a first screen, to cut the pile down to a level we can really analyze. But that's not a "tacit acceptance". (I don't even know what that might mean.)
Yes. After a first cut, we look at the statement of purpose 100% of the time.
And yes, if someone writes a lousy statement with an otherwise good package, it's a bad sign. It tells us that the student just doesn't care to do a good job. Is that someone I want as a grad student? Probably not.
Even if a student has stellar grades, GRE, and a paper in Science but the statement is poorly written, I'm going to think twice compared to a student who might be a bit behind, but seems to have a vision and sells themselves well in the statement.
I serve occasionally on the PhD admissions committee in the computer science department at Stanford. To me, your grades and test scores are less important than your statement of purpose and your recommendation letters. To clarify: the statement of purpose is very, very important. I want to see how you articulate your future plans, how you think about and approach research.
I would add an exception to @RoboKaren's answer. Economics programs tend to place very little weight on statements of purpose. Econ is perhaps a bit different from other disciplines because the first year is all coursework.
In a statement of purpose for an econ program one should highlight relevant previous courses/skills (especially math/stats) and indicate a few areas of interest (e.g. "Macro labor" not a dissertation proposal). Minimize the fluff.
Several economists (including Susan Athey [Stanford] and Jeffrey Smith [Michigan]) have composed helpful guides on grad school admission. I'd bet others have done the same in other disciplines.
The general point that I hoped to make with this post (which was implicit but which I will now make explicit) is that the importance of the personal statement (and the nature of the application in general) is particular to a given discipline.
Why don't economists care about personal statements? First, it is costless for applicants to overstate/misrepresent their interests, talents, background, enthusiasm, etc. Second, given the other materials available to the adcom (see below), even a truthful statement is more or less superfluous. Third, deemphasizing the personal statement reduces the advantage enjoyed by native English speakers in admissions.
What do economists care about, then? By far the most important component is faculty recommendation letters. It means something if a faculty member is willing to say that someone is "the best student they've had in eleventy years in the profession". Suppose a faculty member misstates the qualifications of the applicant and the applicant is admitted (and perhaps offered a stipend). If the applicant fails out, then (1) the adcom will be disinclined from believing the recommender the next time around and (2) there may in certain cases be some informal social/professional consequences (i.e. "Hey remember that time you recommended that one kid and we wasted a spot and 25k on him just so that he could drop out in April of the first year?").
I don't think that this is a perfect system (or that admissions are perfectable) but that's just the way it works and has worked in pretty much any econ department for quite some time. Depending on your view of economics, it either more-or-less works OR goes towards explaining why econ is so messed up(!).
I have to agree with what one of the CS guys wrote above, that in CS we consider the SOP to be extremely important. Also, you should know that MIT does not usually 'tacitly' accept anyone based on only their grades and numbers. Usually, a tacit acceptance comes about because you have a funding source lined up; perhaps you've secured a fellowship or you've established very close contact with a faculty member in MIT or the college you're applying to, and that member has agreed to accept you into the group INCUMBENT on your getting through the general grad school admissions process. At that point, your SOP might not seem as important because it's obvious you've found your focus, and in most cases, the SOP is merely an avenue for you to articulate your focus ('purpose') in a convincing, objective fashion.
