How the DATA Step Identifies BY GroupsProcessing Observations in a BY GroupUsing a Name Literal as the BY-Group VariableHow SAS Determines FIRST.variable and LAST.variableExample 1: Grouping Observations by State, City, ZIP Code, and StreetExample 2: Grouping Observations by City, State, ZIP Code, and StreetExample 3: A Change Affecting FIRST.variable