What type of study primarily focuses on the relationships between existing variables rather than experimenting?

Correlational studies are designed to examine the relationships between existing variables without manipulating them. This means researchers observe and measure these variables as they naturally occur, looking for patterns or associations. For instance, a correlational study might explore how stress levels are related to sleep quality among individuals, determining whether there is a positive or negative relationship between these two variables.

The key characteristic of correlational studies is that they do not involve random assignment or manipulation of variables, which distinguishes them from experimental studies. While longitudinal studies may track changes over time and case studies provide in-depth analysis of a single subject or group, neither primarily focuses on the relationships between variables as correlational studies do. Therefore, correlational studies are the correct choice for understanding how variables relate to each other in a systematic, non-experimental way.