以下是 Chapter 3 "Observational Studies" 的核心内容总结，包含所有重点、Technical Points 和 Fine Points 的结论，严格使用 Markdown 的 LaTeX 公式格式。中英双语呈现：

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核心概念总结

1. 观察性研究 vs. 随机实验

Chapter 3 Summary: Observational Studies

This chapter focuses on causal inference from observational studies, contrasting them with randomized experiments. Key challenges include unmeasured confounding and the need for strong assumptions to estimate causal effects. The core framework involves emulating a target trial using observational data, relying on three identifiability conditions: exchangeability, positivity, and consistency. If these hold, methods like IP weighting or standardization can estimate causal effects. Below, I detail all main points, including conclusions from Technical Points (TP) and Fine Points (FP).