Try to state your research question without describing the sources or data you will use to answer the question.
What sort of analysis do you plan to do? Do you need to data or statistics to illustrate points? Will you be using a stats package, such as R, SPSS, SAS, or Stata, to do analysis?
When you define your topic and unit of analysis, you should look at your research question and ask:
Use specific language when defining your topic. This will help you identify a variable or variables.
Who or what is being described by your variable(s)?
For what point in time do you want to know this about the people, institutions, or products you identified? How often do you want to know it about them?
What part of the world is your research question concerned with?
Are you looking for data collected at regular intervals over time? Identifying what sort of time series may be helpful as you search for data.
* Adapted from Barbara Mento's guide to Finding Data at Boston College