To quote from our admissions web site, the video should be “A short (1-3 minute) video of yourself. Tell us about you and why you want to come to CMU.”

While we are interested in what you say, that is probably redundant with your written Statement of Purpose. The primary reason we ask for a video is to get some additional information on your fluency in English.

For English proficiency, we depend mostly on TOEFL scores and sometimes on your grades in English courses (though those grades may be hard for us to evaluate). But, especially if your spoken TOEFL score is not excellent, this gives you an addition way of showing us that you will have no trouble communicating with faculty and other students and giving presentations during your grad-school career. One of the primary failure modes of our foreign graduate students is that they have so much difficulty with English that they can’t keep up in classes and research, so we take English proficiency very seriously.

We can’t tell too much about your spoken-English ability if you just read a prepared statement (who wrote it?) or deliver a memorized speech word for word (again, who wrote it?). So the most persuasive video is one in which you just speak spontaneously for a couple of minutes. You should know in advance what you want to say, but not the exact words. And yes, we can tell the difference.