The admissions committee is looking at many factors when evaluating applications. The statement of purpose is a key document for the admissions committee. The admissions committee is using all available pieces of information, but the statement of purpose is the central document where you can frame your application and provide context to other information.

Tell us why you are interested in a Ph.D. in Software Engineering from Carnegie Mellon.  If you have been involved in any kind of research, briefly describe the problem, approach, and results.  Describe a problem you are interested in solving as a researcher, and if you have preliminary ideas, how you would go about solving it.  Talk about your general interests in software engineering (beyond the problem above), and if you are interested in working with particular faculty, mention their names. 

Among others, the statement of purpose can be central in evaluating the following important concerns of the admission committee:

• Research aptitude: We are looking for students who will be successful in a research career. Note that what makes students successful in research may be very different from what makes them successful in undergraduate studies or Master programs. Students who pursue a PhD with the wrong expectations may find the experience unrewarding or even frustrating. Strong statements of purposes will help the admissions committee understand that the applicant understands what software engineering research looks like and is motivated to pursue research. Prior research experience (and even publishing experience) can be useful to demonstrate research aptitude and should likely be covered in the statement, but prior research experience is not required. Potential applicants might want to explore research opportunities in their local institutions, participate in summer research internships (e.g. REU programs such as REU-SE), join research mentoring programs (e.g., ICSE-SMeW, OOPSLA-PLMW) or explore recent research publications in the field of interest.
• Area fit: We are looking for students who are interested in software engineering research, where we take a broad perspective of what classifies as software engineering research. Applications that just generically talk of research or applications that mention only other fields (e.g., robotics) without an explicit connection to software engineering tend to be less successful. A strong application shows an understanding of research problems in the field of software engineering or makes a case for exploring a topic (e.g., robotics) from a software engineering perspective. Potential applicants might find it useful to explore research topics at recent academic software engineering conferences (e.g., ICSE or many other venues and journals where our faculty and students publish) and look at this document about how we think about software engineering.
• Interests and potential advisor match: When admitting students, potential advisors often have specific research directions or projects in mind. A good statement of purpose will help us understand in which topics the applicant is interested and how this may match to possible advisors and projects. A good statement of purpose is explicit about interests, though such interest can take many different forms. For example, some applicants may have very specific research ideas or want to work with specific faculty on specific topics; some applicants may have preferences for specific research methods (e.g., quantitative empirical research or formal methods); and others may be broadly interested in software engineering research and may be happy to work on a range of possible topics if the opportunity arises -- all of these are perfectly fine and we routinely admit students with all these profiles. In fact, it is quite common that students have had limited exposure to software engineering research, they enjoyed the research experience, and are interested in doing more similar research but also open to other topics. It is also common that students have some idea about possible projects they would enjoy but are also open to other projects with a similar flavor. Whatever your preferences, it is most helpful to the admissions committee to be honest about preferences (or lack thereof) with regards to topics and research methods. It can be helpful to mention names of potential advisors in the letter if your research interests align with research done by that advisor; in such case please also indicate why you think this advisor would be a good fit or why you are interested in those topics.

The statement of purpose does not need to follow a specific recipe or structure and can cover many other topics that will help you to present yourselves. If your background is unusual, tell us about it and how it bears on your Ph.D. plans. We do not set a page limit, but suggest to keep the statement concise, typically around 2 pages. You can go into more depth in additional optional documents, see below.

Prof. Jonathan Aldrich has some more advice for prospective students writing their statement of purpose.

Submit three letters of recommendation. Recommenders should know you relatively well and be able to evaluate the quality of your previous work. Recommenders that can speak about your research skills are more useful than letters commenting on your performance in coursework. At least two should be from faculty or recent employers. With the online application, letters will be requested and submitted electronically. Paper recommendations will not be accepted.

Letters of recommendation are due by the application deadline. We start reviewing of applications in December very soon after the application deadline and if letters are missing at that point we may have insufficient information to properly evaluate an application.

All applicants are different and have different strengths and backgrounds. Use the application to make a strong case of why you should be accepted to the program. Highlight unusual strengths or perspectives in your application. Although we do not impose a page limit for the statement of purpose, we recommend to keep it concise and focused and to move additional material to an appendix or separately uploaded documents.

Here are some examples:

• Statement of research experience (optional): Some applicants have extensive research experience beyond what can be described in a concise statement of purpose or would like to share additional specific details. An extra statement of research experience can be used to go into more depth on prior research projects.
• Publications (optional): Some applicants have experience publishing their research. In addition to your resume, you can list publications in the application system. You may submit copies of unpublished drafts, papers under review, and thesis documents. Please make the publications available to the committee as PDF uploads or links.
• Statement of industry/government experience (optional): Some applicants have significant experience in industry or government (e.g., internship, co-op, capstone course, consulting, or a full-time position) and that experience may inform their view on research, their research interests, and their motivation to pursue graduate studies. While the statement of purpose should make the connection, an additional optional statement of industry/government experience allows interested applicants to go into more detail. Note, industry/government experience is not required for this PhD program--we encourage applications from all students with strong interests in software engineering, regardless of prior experience.
• Samples of writing, presenting, coding (optional): Some applicants may want to share additional evidence of their communication skills, such as technical or nontechnical writing, slides, or recorded presentations, or their programming skills. Link to those from your resume. 

None of these artifacts are required. We will not evaluate the absence of these artifacts as a negative of your application, but some applications may benefit from strong supporting material, especially when used to explain unusual or exceptional circumstances.