Data collected both across units (e.g., municipalities, states, countries) and over time (e.g. days, months, years)—known as pooled time series—are common in social science. By gaining leverage both across units and over time, this data structure helps us answer important questions that would be difficult if we only looked at a single year (e.g., cross section) or single country (e.g., time series): the relationship between growth and democracy, whether or not the resource curse exists, and how institutions shape political and economic outcomes.