What are you trying to measure or infer in your research project?  Addressing this question leads you to identifying the variables in your work.  Identifying the relevant metrics, and their associated units, is essential in carrying out a meaningful research project.

If your research project involves a study of the relationship between the median income of a neighborhood and the availability of grocery stores then the measured variables involved could be:

1. Median income per zip code ($US)

2. GPS coordinates of grocery stores (degrees latitude, degrees longitude)

3. Area of zip code (square kilometers)

And the calculated variables could be

1. Density of grocery stores (stores per square kilometer)

At this point you'll want to identify what you're measuring and calculating in the course of your work, as well as what units you'll be using in which to measure them.