===== Post hoc observational cohort study =====

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.

==== Key Features ====

  * **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.

==== Limitations ====

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

==== Example ====

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.