The approach in which researchers use existing data to create groups for comparison is called:

Ex post facto research uses already existing data to form comparison groups after the fact.

The groups differ on a variable that has already occurred and cannot be manipulated.

Operational refers to defining variables in measurable terms, not a design type.

Experimental research actively manipulates the independent variable and assigns participants.

Quasi-experimental research manipulates a variable but lacks full random assignment.

Ex post facto designs can suggest associations but not firm causal conclusions.