A 'post hoc observational cohort study' is a type of observational cohort study in which a retrospective analysis is conducted using data that was already collected, usually for a different original purpose.

• Observational: No intervention is applied; researchers analyze associations within existing data.
• Cohort: The study population is a defined group followed over time.
• Post hoc: The analysis was not pre-specified in the original study protocol; it is performed after data collection.
• Retrospective: Uses previously gathered data such as medical records, registries, or clinical trial datasets.
• Exploratory: Aims to generate hypotheses rather than establish causation.

• Increased risk of bias (e.g., selection, confounding).
• Associations found are not necessarily causal.
• Limited to the quality and completeness of existing data.

A clinical trial dataset on aneurysmal subarachnoid hemorrhage, initially collected to study vasospasm, could later be analyzed post hoc to explore whether initial systolic blood pressure correlates with 3-month outcomes.