All patients aged 18 years and over who arrived at participating centres between 1 January 2023 and 31 December 2024.

In this first phase of the project, representatives of doctors and nurses from participating hospitals will be asked to express their opinion on content validity. This refers to the accuracy with which a measurement tool covers all aspects of the construct under examination. Its measurement is generally based on the judgment of experts in the field, who are asked to provide feedback on how closely the measurement tool corresponds to the different domains associated with the construct.

The evaluation will be conducted according to the consensus conference approach, an approach that reduces the risk of excessive subjectivity, intrinsic to content validity. This evaluation will lead to a possible refinement of the individual criteria that make up the algorithm.

In this second phase, the criterion validity of the algorithm developed in the previous phase will be examined. Criterion validity measures the relationship between the results obtained with the test in question and those obtained with a consolidated reference standard (the criterion, or gold standard) that measures the same construct.

Once perfected, the algorithm will be applied to all eligible patients arriving at the ER of participating hospitals within a 24-month period (from January 2023 to December 2024). This means that, for all hospitalized patients, it will be possible to obtain data on the appropriateness of this choice and monitor the evolution of this data over time.

In this phase, an attempt will be made to obtain the data necessary to evaluate the appropriateness of discharges from the ED for eligible patients who were discharged between January 2023 and December 2024. Data will be collected on all accesses in the period considered in order to identify any multiple patient accesses (return visits) and the related presentation tables.