A PhD dissertation is a lengthy, formal document that argues in defense of a particular thesis. The research performed to support the thesis must be original and substantial, and the dissertation must show it to be so. In particular, a dissertation highlights original contributions. The scientific method means starting with a hypothesis and then collecting evidence to support or deny it. The most difficult aspect of writing a dissertation consists of organizing the evidence and associated discussions into a coherent form. The essence of a dissertation is critical thinking, not experimental data: analysis and concepts form the heart of the work. A dissertation concentrates on principles: it states the lessons learned, and not merely the facts behind them. In general, every statement in a dissertation must be supported either by a reference to published scientific literature or by original work. Moreover, a dissertation does not repeat the details of critical thinking and analysis found in published sources; it uses the results as fact and refers the reader to the source for further details. Each statement in a dissertation must be correct and defensible in a logical and scientific sense; the discussions in a dissertation must satisfy the most stringent rules of logic applied to mathematics and science.
If your dissertation is like most, it will only be read by your committee and some other . candidates seeking to build on your work. As such, it does not need to be a masterwork of literature, nor does it need to solve a long-standing problem in computing. It merely needs to be correct, to be significant in the judgement of your committee, and it needs to be complete. We will all applaud when you change the world after graduation. And at that you will find that many well-known scientists in CS have made their careers in areas different from their dissertation topic. The dissertation is proof that you can find and present original results; your career and life after graduation will demonstrate the other concerns you might have about making an impact.
In her dissertation on America's China policy under Truman and Nixon, entitled "Clearer Than Truth," Crowley, whose . is in international relations, lifted multiple passages from Eric Larson's 1996 book, "Casualties and Consensus: The Historical Role of Casualties in Domestic Support for . Military Operations." She also repeatedly plagiarized James Chace's 1998 book, "Acheson: The Secretary of State Who Created the American World," as well as a 1982 book by Yale's John Lewis Gaddis called "Strategies of Containment: A Critical Appraisal of American National Security Policy during the Cold War." Crowley's dissertation also contains passages taken from a 1996 book by Thomas Christensen of Princeton, Useful Adversaries: Grand Strategy, Domestic Mobilization, and Sino-American Conflict, 1947-1958.