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. Measured variables are values which are directly measured by an observer, such as using a ruler for length or a thermometer for temperature.
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)
At this point you'll want to identify what you're measuring in the course of your work, as well as what units you'll be using in which to measure them.