Epidemiology: Measurements in Epidemiology

The first blog in the series was Epidemiology-1.

Incidence:

It is number of NEW cases occurring in a defined population during a specified period of time.
Formula of Incidence:
(no.of NEW cases in a defined population during a specified time period )/(Population at risk during the same time period) X 1000

Prevalence:

It refers to ALL the cases (old and new) present in a population at a given period of time.
Formula of prevalence:
(no.of All cases in a defined population during a specified time period )/(mid-interval population at risk) X 1000

Uses of ‘Incidence’:
Incidence can be used:
1. As the indicator of health services as it usually responds earlier than prevalence
2. In research for ‘Risk factors’ or cause
3. For checking the efficacy of therapeutic of preventive measures e.g. comparing the incidence of disease among the vaccinated and the un-vaccinated

Uses of ‘Prevalence’:
Prevalence can be used for:

1. Measuring the disease burden in the community
2. For prioritizing the health problems of the population, for example, the amounts of TB, Leprosy etc. in India have made the government give them a higher priority
3. It is needed for planning health services e.g. number of beds needed for the particular disease, amount of medicines needed in the store.

Measurements of mortality:

1. Crude death rate (CDR)
2. Specific death rates
3. Case fatality ratio (CFR)
4. Proportional mortality rate (PMR)
5. Survival rates
6. Standardized death rate (SDR)
7. Standardized mortality ratio (SMR)

Formulas for the above death rates:

CDR: number of deaths from any cause, per 1000 population in one year, in a defined population
(no. of deaths in one year in a specified area)/(mid-year population of the specified area during the same year) X 1000

Specific death rates:

Cause specific death rate e.g. Tuberculosis death rate

((no. of deaths due to the cause in one calender year in a specified area)/(mid-year population of the specified area during the same calender year) )X 1000

Age specific death rate e.g. death rate in the reproductive age group

((no. of deaths in the age group 15-44 yr,in one year in a specified area)/(mid-year population of the 15-44 yr old,during the same year)) X 1000

Death rate for a month e.g. in January

((no. of deaths in January X 12)/(mid-year population ) )X 1000

Weekly death rate

((no. of deaths in the given week X 52)/(mid-year population ) )X 1000

Proportional mortality ratio

((no. of deaths due to the cause (or in a particular group))/(TOTAL no.of Deaths during the same year in the same population)) X 1000

Case fatality ratio (CFR):

((no. of deaths due to the particular disease in one year in a specified area)/(no.of cases diagnosed with the disease in the same year and area)) X 1000

Note: you may be asked to differentiate between ‘Cause-specific death rate’, ‘Proportional mortality ratio’ (PMR) and ‘Case Fatality ratio ‘(CFR). Hint: Pay attention to the denominator

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Epidemiology: Measurements in Epidemiology