Which among the following is false about case fatality rate?

. case fatality rate represents the killing power of a disease.it is simply the ratio of deaths to cases.the time interval is not specified.it is usually used in the case of acute infections such as food poisoning,cholera,measles etc.its usefulness to chronic diseases is limited.because the period from onset to death is long and variable.the case fatality rate for the same disease may very in epidemics.it is closely related to VIRULENCE. REF:park&;s textbook ,ed22,pg no 55