An effective thesis expresses the main idea of a paper in a complete, declarative sentence. A declarative sentence makes a statement and is not a question. Although using questions to generate ideas helps the writer develop strategies for addressing critical issues in the thesis and the paper, the thesis statement itself should never end in a question mark. "What causes economic collapse?" is a sound research question, but it is not a thesis. "Economic collapse may be predicted by trends in unemployment rates," however, is a thesis statement.
An effective thesis makes a statement and takes a position on a topic. "Some people think the legal voting age should be raised, while others do not" provides information but does not state a position. "Although some people think the legal voting age should be raised, eighteen-year-olds should have the right to vote" states a position on the topic. A well-written thesis guides readers toward final conclusions of the paper. The thesis indicates the direction and purpose of the paper, expresses the author's argument, includes sound, logical reasons and must be debatable. (See References 2, Some Caveats and Examples)
An effective thesis is neither too broad nor too narrow in scope. A thesis statement that is too broad makes sweeping generalizations that are difficult to prove within the parameters of the paper. "Education needs improvements" is too broad a statement. In contrast, writers may have difficulty finding enough information to support a thesis that is too narrow. "To help combat high failure rates among college freshmen, public schools should help students develop study skills" offers adequate focus and reflects the author's intent.
An effective thesis is supported with verifiable facts. If writers cannot substantiate their claims with information from valid sources, they should revise the thesis statement. Thesis statements evolve during the writing process. A working thesis allows writers to revisit, revise, restructure and redirect the main point as necessary as they uncover more information about the topic in the research process. Revising the thesis statement in these cases indicates writers are gaining valuable knowledge through research and critical thinking.