In epidemiology, a case fatality risk or case-fatality ratio – is the proportion of deaths from a certain disease compared to the total number of people diagnosed with the disease for a particular period. A CFR is conventionally expressed as a percentage and represents a measure of disease severity. CFRs are most often used for diseases with discrete, limited-time courses, such as outbreaks of acute infections. A CFR can only be considered final when all the cases have been resolved. The preliminary CFR, for example, during an outbreak with a high daily increase and long resolution time would be substantially lower than the final CFR.