This article provides a guide on how to use clinical decision rules (CDRs) in clinical practice, focusing on the Ottawa ankle rules. The article discusses the development, validation, and impact analysis of CDRs, emphasizing the importance of rigorous methodology and evidence-based evaluation. CDRs are clinical tools that quantify the contributions of various factors in diagnosis, prognosis, or treatment response. They aim to simplify and improve the accuracy of clinical assessments. The Ottawa ankle rules are a CDR used to determine when ankle radiographs are necessary for patients with ankle injuries. These rules have been validated in multiple studies and are considered level 1 CDRs, meaning they are reliable and can be implemented in clinical practice. The rules help reduce unnecessary radiographs, thereby decreasing costs without compromising patient care. The article outlines the three steps involved in developing and testing a CDR: creation of the rule, testing or validating the rule, and assessing the impact of the rule on clinical behavior. Validation is crucial to ensure that the rule works consistently across different populations and settings. The Ottawa ankle rules have been validated in various clinical settings, demonstrating their reliability and effectiveness. The article also discusses the importance of impact analysis, which involves evaluating whether the use of a CDR leads to changes in clinical behavior and improved patient outcomes. A study on the Ottawa ankle rules showed that their implementation reduced the number of radiographs ordered without increasing adverse outcomes, supporting their use in clinical practice. Finally, the article emphasizes the need for local implementation strategies to ensure that CDRs are effectively adopted by clinicians. Even though the Ottawa ankle rules are level 1 CDRs, their successful implementation requires appropriate training and support to change provider behavior. The article concludes that CDRs can improve clinical judgment, change behavior, and reduce unnecessary costs while maintaining quality of care.This article provides a guide on how to use clinical decision rules (CDRs) in clinical practice, focusing on the Ottawa ankle rules. The article discusses the development, validation, and impact analysis of CDRs, emphasizing the importance of rigorous methodology and evidence-based evaluation. CDRs are clinical tools that quantify the contributions of various factors in diagnosis, prognosis, or treatment response. They aim to simplify and improve the accuracy of clinical assessments. The Ottawa ankle rules are a CDR used to determine when ankle radiographs are necessary for patients with ankle injuries. These rules have been validated in multiple studies and are considered level 1 CDRs, meaning they are reliable and can be implemented in clinical practice. The rules help reduce unnecessary radiographs, thereby decreasing costs without compromising patient care. The article outlines the three steps involved in developing and testing a CDR: creation of the rule, testing or validating the rule, and assessing the impact of the rule on clinical behavior. Validation is crucial to ensure that the rule works consistently across different populations and settings. The Ottawa ankle rules have been validated in various clinical settings, demonstrating their reliability and effectiveness. The article also discusses the importance of impact analysis, which involves evaluating whether the use of a CDR leads to changes in clinical behavior and improved patient outcomes. A study on the Ottawa ankle rules showed that their implementation reduced the number of radiographs ordered without increasing adverse outcomes, supporting their use in clinical practice. Finally, the article emphasizes the need for local implementation strategies to ensure that CDRs are effectively adopted by clinicians. Even though the Ottawa ankle rules are level 1 CDRs, their successful implementation requires appropriate training and support to change provider behavior. The article concludes that CDRs can improve clinical judgment, change behavior, and reduce unnecessary costs while maintaining quality of care.