Case fatality rate , Mortality Rate

 Case fatality rate, also called case fatality ratio,:the proportion of people who die from a specified disease among all individuals diagnosed with the disease over a certain period of time

 

Case fatality rate is calculated by dividing the number of deaths from a specified disease over a defined period of time by the number of individuals diagnosed with the disease during that time; the resulting ratio is then multiplied by 100 to yield a percentage

 mortality rate, another measure of death for a given population

As an example, consider two population. One population consists of 1,000 people; 300 of these people have the specified disease, 100 of whom die from the disease. 

In this case, the mortality rate for the disease is 100 ÷ 1,000 = 0.1, or 10 percent. 

The case fatality rate is 100 ÷ 300 = 0.33, or 33 percent. 

The second population also has 1,000 people; 50 people have the disease and 40 die from it. Here the mortality rate is 40 ÷ 1,000 = 0.04, or 4 percent; 

the case fatality rate, however, is 40 ÷ 50 = 0.8, or 80 percent. 

The incidence of death from the disease is higher in the first population, but the severity of disease is greater in the second

mortality rate, another measure of death for a given population

mortality rate is calculated by dividing the number of deaths by the population at risk during a certain time frame.

Prevalence, in epidemiology, the proportion of a population with a disease or a particular condition at a specific point in time (point prevalence) or over a specified period of time (period prevalence). 

Prevalence is often confused with incidence, which is concerned only with the measure of new cases in a population over a given interval of time.

For prevalence, the numerator is the number of existing cases or conditions, and the denominator is the total population or group. 

For example, the prevalence of type 2 diabetes among children age 2 to 12 equals the number of children age 2 to 12 years with type 2 diabetes divided by the total number of children within that age range.

Prevalence is especially useful to health system planners and public health professionals. Knowledge of the disease burden in a population, whether global or local, is essential to securing the resources required to fund special services or health-promotion programs. For instance, the director of a nursing home must be able to measure the proportion of seniors with Alzheimer disease in order to plan the appropriate level of services for the residents. Legislators and public health professionals require population statistics in order to prioritize funding for health programs, such as those aimed at obesity reduction or smoking cessation. National- and state-level prevalence of behaviours and diseases is usually calculated using data collected systematically from the population through major health surveys, such as the National Health and Nutrition Examination Survey (NHANES) in the United States.

Prevalence is related mathematically to incidence. When the incidence of disease is stable over time, such as in the absence of epidemics or changes in treatment effectiveness, prevalence (P) is the product of the incidence (I) and the average duration (D) of the disease or condition, or P = I × D. More complex mathematical relationships exist between incidence and prevalence when those assumptions cannot be met.

Incidence, in epidemiology, occurrence of new cases of disease, injury, or other medical conditions over a specified time period, typically calculated as a rate or proportion. Examples of incident cases or events include a person developing diabetes, becoming infected with HIV, starting to smoke, or being admitted to the hospital. In each of those situations, individuals transition from an occurrence-free state to an occurrence.

Incidence versus prevalence

Incidence contrasts with prevalence, which includes both new and existing cases. For example, a person who is newly diagnosed with diabetes is an incident case, whereas a person who has had diabetes for 10 years is a prevalent case. For chronic diseases, such as diabetes, a person can have an incident case just once in a lifetime. For diseases that can be resolved (e.g., the common cold), a person can have multiple incidences over his or her lifetime.
The study of incident cases provides information about the etiology (or cause) of a disease and its outcome. It also allows researchers to determine the risk factors for a disease or other medical condition. The study of prevalent cases, on the other hand, combines the study of new and surviving cases, making it unclear as to whether risk factors are the causes of new cases or causes of survival.

Incidence proportion and incidence rate

Incidence can be measured as a proportion or as a rate. Measured as a proportion, it quantifies the risk of an occurrence in a given time period. Measured as a rate, it quantifies the number of new cases in a population over time. Thus, to calculate incidence, three elements must be defined: (1) the number of new cases, (2) the population at risk, and (3) the time period.
For incidence proportion, the numerator is the number of new cases of a disease or condition that occur during a given time period, while the denominator is the total population at risk during the defined study period. To accurately measure incidence proportion, all individuals at risk for the outcome under study must be followed during the entire study period (or until experiencing the outcome). Because complete follow-up is required to directly compute incidence proportion, it is usually only calculated for studies with a short follow-up period. As an example, on a seven-day cruise, 84 of 2,318 passengers report to the ship’s infirmary with gastrointestinal illness. The incidence of disease on the ship would equal 84 new cases of illness divided by 2,318 total passengers at risk, resulting in an incidence proportion of 4 percent over the seven-day period.
The incidence rate numerator is likewise the number of new cases. The denominator, however, is the total person-time, or the amount of time that all at-risk persons were observed. For example, the hypothetical incidence rate of breast cancer among women age 40 or older equals 32 women with breast cancer divided by 3,896 person-years (persons per year) of follow-up, which is equivalent to 821 per 100,000 at-risk persons per year.
Because incidence rate includes persons entering and leaving study populations, whereas incidence proportion assumes that those individuals were disease-free, it generally is more accurate than incidence proportion for long-term studies. Hence, an accurate measure of incidence, whether incidence proportion or incidence rate, requires a precise definition of the denominator. Because incidence is a measure of new cases during a given time period, it is important that those persons in the denominator be at risk. They should not have a history of the disease in question if it is a chronic disease, nor should they otherwise not be able to develop a new case of disease (e.g., women cannot get prostate cancer). An additional consideration for the use of incidence rate is that it assumes a constant probability of disease, which may not reflect actual probability, particularly for conditions whose risk increases with age.