The purpose statement in a dissertation is similar to an introduction. It serves as an overall statement of intent both for the readers and the person writing the dissertation. It also acts as a kind of navigational tool for students to help them as they negotiate their way through writing the document. The first part of a purpose statement should determine the type of study to be used in the dissertation. Is it qualitative or quantitative? Is it an experiment or a correlational analysis? The statement should state the purpose such as: "The purpose of this quantitative study is to...."
The next part of a purpose statement should be to clearly lay out the topic of the dissertation and the method through which it will be studied. The topic should be clearly stated in the plainest terms possible, such as: "This longitudinal study is to document the challenges to development of food crops in New England over a five-year period." This serves as the road map for the writer's future and clearly states to the reader what exactly he is studying.
The population and setting of the study should then be stated in the purpose statement such as: "The purpose of this quantitative, correlational study was to test whether an increase in the quality of the leader-member exchange correlates with an increase in job satisfaction for employees working in a national hotel chain." Here clearly the focus of the study is narrowed down to a definable group -- in this case "employees working in a national hotel chain."
In addition to these three elements a purpose statement may contain other information. There may be a secondary purpose to the primary purpose. There may be a brief description of the quantitative variables. There may be a definition of the methodology used, there may be a note about the role of the researcher and the participants in a qualitative study and there may be a note connecting the purpose to other aspects of the study.