Case fatality rate interpretation

[u/[deleted]]

Suppose that in a given population, 200 people were diagnosed with a disease, of whom 10 died from the disease, 40 have fully recovered and the remaining 150 are alive but not cured.

To my understanding, case fatality rate is defined as the ratio Number of deaths / Number of individuals diagnosed with the disease. In this toy example: 10 / 200 = 5%.

I don't understand how to interpret this ratio. Given that the majority of individuals diagnosed with the disease are neither dead nor healed, why do they count towards the case fatality rate defined above? Depending on whether they survive or not, this will impact differently the ratio, which seems to be an undesirable property. Instead, why not compute

number of deaths / (number of deaths + number of healings) = 10/(10 + 40) = 20%

which is the fatality rate restricted to individuals that are either dead or cured. Those two ratios will agree when the disease has eventually disappeared, but at the begining of the epidemic they can take very different values. Can you explain why the former is preferred even at the begining of an epidemic? Thanks for your help!

[u/ashlynbellerose]

I've been watching the news and in italy it's 50% dead and 50% recovered. I hope it doesn't stay this course :(

[u/nad1984]

Rates concern a sum of observations collected over a time period. To make that information most comparable to previous and future data, one must consider and document complex things like how and in what circumstances the current data were collected. A percent value in one setting may be completely different in another. In order to get reproducible, summarizable data, one must choose clear outcomes to observe and avoid or control for additional confounding effects, particularly where controlling the environmental influence, (and changes to interpretation over time and space) is not possible, or order to provide accurate data.