4.2 Formulate your question
Previously in this book, we have discussed the importance of properly formulating a question. Formulating a question can be a useful way to guide the exploratory data analysis process and to limit the exponential number of paths that can be taken with any sizeable dataset. In particular, a sharp question or hypothesis can serve as a dimension reduction tool that can eliminate variables that are not immediately relevant to the question.
For example, in this chapter we will be looking at an air pollution dataset from the U.S. Environmental Protection Agency (EPA). A general question one could as is
Are air pollution levels higher on the east coast than on the west coast?
But a more specific question might be
Are hourly ozone levels on average higher in New York City than they are in Los Angeles?
Note that both questions may be of interest, and neither is right or wrong. But the first question requires looking at all pollutants across the entire east and west coasts, while the second question only requires looking at single pollutant in two cities.
It’s usually a good idea to spend a few minutes to figure out what is the question you’re really interested in, and narrow it down to be as specific as possible (without becoming uninteresting).
For this chapter, we will consider the following question:
Do counties in the eastern United States have higher ozone levels than counties in the western United States?
As a side note, one of the most important questions you can answer with an exploratory data analysis is “Do I have the right data to answer this question?” Often this question is difficult to answer at first, but can become more clear as we sort through and look at the data.