Some Key Epidemiological Definitions / Formulas:

 

  1. Crude Death Rate = No. deaths in a year / Average (midyear) population

 

  1. Cause Specific Death Rate = No. Cause-Specific deaths in a year / Average mid-year population

 

  1. Age-Specific Death Rate = No. deaths among persons in a particular age group / Average mid-year population in specified age group

 

  1. Proportional Mortality Rate = No. deaths from specific cause over specific time period / Total deaths in same period

 

  1. Case Fatality Rate = No. deaths due to specific disease / No. cases of specified disease

 

  1. Survival Rate = No. cases alive at end of a specified period of time / No. of cases alive at start of period

 

  1. Maternal (puerperal) Mortality Rate = No. deaths from puerperal causes in a year / No. of live births same year

 

  1. Infant Mortality Rate = No. infant deaths during year / No. of live births in same year

 

  1. Neonatal Mortality Rate = No. deaths in a year of children younger than 28 days old / No. of live births in same year

 

  1. Fetal Death Rate = No. fetal deaths during year / No. of live births and fetal deaths in same year

 

  1. Perinatal Mortality Rate = No. fetal deaths 28 weeks or more and infant deaths younger than 7 days of age during year / No. of live births and fetal deaths 28 weeks or more gestation in same year

 

  1. Standard Mortality Rate = No. of deaths in a cohort of exposed persons / The expected no. of deaths for persons fitting the cohort membership characteristics as based upon historic mortality data or comparisons to comparable populations. 

 

  1. Standardized Mortality Ratio: This is the relative measure of the difference in risk between the exposed and unexposed populations in a cohort study. The SMR is similar to the relative risk in both definition and interpretation. This measure is usually standardized to control for any differences in age, sex, and/or race between the exposed and reference populations. It is frequently converted to a percent by multiplying the ratio by 100.  An SMR > 1.0 implies that the rate is greater for the population of interest compared to the standard population. An SMR < 1.0 implies that the death rate is lower for the population of interest compared to the standard population.

 

  1. Standardized Incidence Ratio: This is the relative measure of the difference between th observed number of new cases of a disease as compared to the expected number of new cases, where the number of expected cases is based upon the disease incidence in a comparison population.

 

  1. Proportional Incidence Rate (PIR) as Applied to Cancer = The ratio of observed cases of specific types of cancer divided by the expected number of cancers multiplied by 100 defines the proportional incidence ratio (PIR) - the measure of risk reported; five year age-bands were used to adjust for age. The PIR can be expressed as PIR = (R/ E) x 100 where R = observed number of cancer cases in the CF group of interest and E = expected number of cancer cases in the cancer group of interest. The expected number of cancer cases (E) is obtained as follows:

E = i ti (ri* / ti*)

ri = number of cases of the cancer group of interest in the age group i in the standard population

ti* = number of cases of cancer (all sites) in the age group of the study group

ti = total number of cases of cancer in age group i of the group