A system for use in evaluating a clinical guideline, comprising: - a guideline data interface configured to access guideline data defining a machine readable version of the clinical guideline in the form of a decision tree (200, 202) comprising at least one node and a decision rule associated with the node, wherein the decision rule comprises at least one variable representing a biomedical quantity; - a patient data interface configured to access patient data (042, 300); - a processor configured to: - extract the biomedical quantity from the patient data using an ontology which defines concepts and their relationships in a medical domain of the clinical guideline and which thereby relates the variable of the decision rule to the patient data; - generate display data for display to the user, wherein the display data is indicative of the biomedical quantity; - obtain a user confirmation indicating a validity of the extracted biomedical quantity; - upon receiving the user confirmation, determine an outcome of an evaluation of the decision rule on the basis of the biomedical quantity; - if the biomedical quantity cannot be extracted from the patient data using the ontology, automatically generate the display data to present a view (400, 410) of the patient data to the user to enable the user to determine the biomedical quantity from the view. - a guideline data interface configured to access guideline data defining a machine readable version of the clinical guideline in the form of a decision tree (200, 202) comprising at least one node and a decision rule associated with the node, wherein the decision rule comprises at least one variable representing a biomedical quantity; - a patient data interface configured to access patient data (042, 300); - a processor configured to: - extract the biomedical quantity from the patient data using an ontology which defines concepts and their relationships in a medical domain of the clinical guideline and which thereby relates the variable of the decision rule to the patient data; - generate display data for display to the user, wherein the display data is indicative of the biomedical quantity; - obtain a user confirmation indicating a validity of the extracted biomedical quantity; - upon receiving the user confirmation, determine an outcome of an evaluation of the decision rule on the basis of the biomedical quantity; - if the biomedical quantity cannot be extracted from the patient data using the ontology, automatically generate the display data to present a view (400, 410) of the patient data to the user to enable the user to determine the biomedical quantity from the view. - extract the biomedical quantity from the patient data using an ontology which defines concepts and their relationships in a medical domain of the clinical guideline and which thereby relates the variable of the decision rule to the patient data; - generate display data for display to the user, wherein the display data is indicative of the biomedical quantity; - obtain a user confirmation indicating a validity of the extracted biomedical quantity; - upon receiving the user confirmation, determine an outcome of an evaluation of the decision rule on the basis of the biomedical quantity; - if the biomedical quantity cannot be extracted from the patient data using the ontology, automatically generate the display data to present a view (400, 410) of the patient data to the user to enable the user to determine the biomedical quantity from the view.