This article, part of the Users' Guides to the Medical Literature series, focuses on how to use articles about clinical decision rules (CDRs) in evidence-based medicine. The scenario involves a medical director of an inner-city emergency department who wants to implement the Ottawa ankle rules to reduce unnecessary radiological procedures for minor trauma. The article outlines the process of developing and validating CDRs, emphasizing the importance of statistical methods and rigorous validation. It also discusses the hierarchy of evidence for CDRs, categorizing them into levels based on their derivation, validation, and impact on clinical behavior. The article highlights the need for clinicians to assess the strength of CDRs and their potential impact before implementing them. Finally, it provides an example of how the Ottawa ankle rules were validated and their impact on reducing unnecessary radiographs was assessed, leading to their classification as a level 1 CDR. The article concludes by emphasizing the importance of efficient implementation strategies to ensure the effectiveness of CDRs in changing clinical behavior and reducing costs without compromising quality care.This article, part of the Users' Guides to the Medical Literature series, focuses on how to use articles about clinical decision rules (CDRs) in evidence-based medicine. The scenario involves a medical director of an inner-city emergency department who wants to implement the Ottawa ankle rules to reduce unnecessary radiological procedures for minor trauma. The article outlines the process of developing and validating CDRs, emphasizing the importance of statistical methods and rigorous validation. It also discusses the hierarchy of evidence for CDRs, categorizing them into levels based on their derivation, validation, and impact on clinical behavior. The article highlights the need for clinicians to assess the strength of CDRs and their potential impact before implementing them. Finally, it provides an example of how the Ottawa ankle rules were validated and their impact on reducing unnecessary radiographs was assessed, leading to their classification as a level 1 CDR. The article concludes by emphasizing the importance of efficient implementation strategies to ensure the effectiveness of CDRs in changing clinical behavior and reducing costs without compromising quality care.