This document discusses the concept of primary and secondary data sources in healthcare. It explains how primary data is collected in real-time through observations, surveys, experiments, interviews, and questionnaires, while secondary data relies on literature reviews or reviewing primary data. The document also highlights the importance of both types of data in evaluating patient health conditions and provides examples of different types of secondary data sources such as databases, registries, and indices. Additionally, it explores the reliability of secondary data and how it can be validated through the reliability of the chosen primary data.