The first step in developing a Ph. D. prospectus is to determine which field of study and academic advisory team most closely align with your interests. Programs vary from university to university, but most theses fall under one of the following categories: Microeconomic Theory, Econometrics, Industrial Organization, International Economics, Labor Economics, Macroeconomics or Public Economics. These broad studies encompass smaller fields of interest; for example, health economics is a part of public economics as it relates to the government's role in providing for the people. While econometrics is focused on formulaic methods of analysis, other fields put more emphasis on the theoretical aspects of economics, either through modeling or historical interpretation. Labor economics is a particularly applicable field; many projects work toward changing public policy by gathering and interpreting information about different markets.
Most post-graduate programs encourage students to study the research of others as well as conduct their own research. Your thesis will require some of both, but the focus may slant more heavily in one direction depending on your topic. For example, a thesis weighing the benefits of social protection or economic independence in various situations will require extensive modeling and cost/benefit analysis. Conversely, a study of the development of the American economy will be structured primarily around original studies and information, current research and understanding of the topic and your own interpretation of the relationship between past and current economic systems.
Once you have a general idea of the topics and research methods you are comfortable with, the next step is to narrow your topic to one area which can be covered exhaustively; ideally you should choose an angle that has not been widely addressed. Potential distinguishing ideas include the study of a particular geographic area or demographic, the development of an economic belief or system over a certain period of time, the application of a certain policy or taxation and results or the understanding of a correlation (for instance, economic growth and social change).
If you are struggling with finding a topic that interests you, consider looking into current issues. Not only is there an abundance of research available, your chances of eventual publication may increase with a topical thesis. Read the economic and business section of major newspapers for ideas. Some ideas to consider include the effect a recession has on budget-making, mortgage and home-owning solutions, the federal reserve and international trade and unemployment and productivity studies.