Measures of (injury and illness) occurrence: a primer on epidemiological concepts and terminology for authors.

Since the beginning of Science and Medicine in Football (2017) as a standalone journal, we have experienced a rise in submissions in the area of epidemiology for injury and illness. Unfortunately, we have also noticed a pattern of inaccurate and confusing terminology when referring to measures of injury and illness occurrence, and when communicating risks. This can be partially explained, because the use of various terms as synonyms and ambiguous definitions has generated confusion and debate even within epidemiology (ElandtJohnson 1975; Bhopal 2016; Spronk et al. 2019). With this editorial, we aim to help authors, and our readers, by providing a very brief primer on the measures of occurrence used in epidemiology. A first distinction among the measures of disease frequency is between incidence and prevalence. For simplicity, we assume the existence of a data sample with data on individuals (e.g., football players) belonging to a group of interest, which we will refer to as ‘study population’. In the study population, certain health-related events or outcomes (e.g., illness or injury) are registered, and such an event or outcome will be referred to as ‘event’. Individuals in the study population that have or (over time) experience an ‘event’ are referred to as ‘cases’.