[Clinical research: why we must mater the essentials].

Learning clinical research methods, we must master the essentials. Essentials are the theories, principles and methods that make a discipline a unique discipline, without which the discipline will no more exist. Essentials are those things that are long-lasting and reliable, those that address the most important issues, those that are most frequently used in practice, and those that normally should not be compromised. The core underlying theories of epidemiological research are causation and errors control. The essentials often have profound theoretical underpinnings and difficult to grasp. We will truly believe the theories and observe the principles only when we have truly grasped them.