What observation technique involves studying one person in depth to reveal universal principles?

The observation technique that involves studying one person in depth to reveal universal principles is a case study. This method allows researchers to gather detailed qualitative data about an individual, providing insights that may apply to broader populations or conditions. By thoroughly investigating a single subject's experiences, behaviors, and context, researchers can identify patterns and potential principles that might be relevant across various cases. This depth of analysis can uncover nuances and complexities that broader methods like surveys or experimental studies might overlook.

In contrast, surveys typically collect data from large groups to find trends, while experimental studies manipulate variables to assess cause-and-effect relationships. Correlational studies examine relationships between variables but do not delve deeply into individual cases. Therefore, the case study approach is particularly valuable when the goal is to gain a comprehensive understanding of complex phenomena through an in-depth lens.