When creating a thesis statement, its topic must be narrow enough that you can discuss it within the time and space that you have available for your assignment. That is because a broad topic will have too many options and too much material to cover for you to have any discernable direction to follow. One way to start narrowing your thesis is by questioning its topic with who? what? where? when? why? how? and which? Performing research can help you figure out which questions are most useful, and using one or more of them together can help you narrow a thesis quite well. However, be careful not to make your thesis's topic too narrow, as you might not be able to find enough to say about it. For an example of the questioning process, see the following example:
Broad thesis statement: Tree frogs need to be studied. Broad topic: tree frogs
Question: what kind of tree frogs? Answer: gray tree frogs
Question: gray tree frogs located where? Answer: gray tree frogs in Oklahoma
Question: why do these gray tree frogs in Oklahoma matter? Answer: endangered gray tree frogs in Oklahoma
Narrowed thesis statement: Endangered gray tree frogs in Oklahoma need to be studied.
Bear in mind, this thesis could be narrowed even further, should its author want to do so. However, it is up to the essay's writer to decide just how narrow is narrow enough, based on their research of the topic.
For more information on how to narrow a thesis, please see the following links: