Start with this sentence and fill in the blank: "The purpose of this study is ... ." This opening will help you clarify in your own mind exactly how your purpose statement needs to be written.
State the purpose in a way that an intelligent person who is well-read, but uninformed in your area of study, would understand it. If the purpose of the study isn't clear to you, it won't be to your reader either.
Provide brief and specific background information of the purpose. This will be revisited later, so don't dwell on it at this point. A sentence or two should cover it.
Incorporate the rationale for the study in your purpose statement so your reader knows exactly why you want to conduct the study for your dissertation. Some committees prefer that the "rationale" and the "purpose" are in separate sections, so check first.
Present the purpose statement within the context of the dissertation. Discuss the reasons that support the theoretical or conceptual framework you have chosen.
Identify the main ideas or central concepts of the purpose for your study. Make a judicious choice between operational or descriptive definitions when defining terms.
Introduce the methodology you will be using in your purpose statement. Present a logical rationale for your choice.
Focus on the points outlined as you write a draft of your purpose statement. Review your draft and make sure you addressed all the issues in an intelligent manner.
Leave the draft for a day or more and then come back to it. Read the purpose statement draft aloud to yourself and think about how it sounds to other people. Correct any typing or grammar mistakes.
Review your purpose statement with your dissertation adviser and ask her if she has any ideas or suggestions of how you could improve it. Print a copy of the final purpose statement and give her a copy for your file.