Overview of existing literature
They organize the literature according to two variables, both of which concern researchers' methods.
First, they contrast studies that employ data drawn from a single time period with research that uses longitudinal data.
Next, they parse the literature according to level of observation: aggregate- vs. individual-level data.
It is common for authors to begin their review at 30,000 feet as Castiglione and Infante do here. They will zoom in on select works as the literature review proceeds.