Rate or Ratio?

Mortality Rates

Vital Statistics analyzes vital events occurring in the life of an individual. Mortality, fertility and migration are some examples of such vital events. Vital statistics comprises of measures such sex ratio, crude birth rate, crude death rate etc. But terms Rate and Ratio are some what ambiguous. The definition of ratio here doesn't seem to confirm with the traditional mathematical definition which is as follows. A ratio is written as " a to b" or "a:b" and is expressed as quotient of two numbers.  Rate is mathematically defined as quantity measured with respect to another quantity. So rates are always defined in terms of  two units like miles per hour for speed or number per milliliter of water for bacterial growth. 

This ambiguity is illustrated with following example. Sex ratio is defined as number of males per 100 females.  So if sex ratio is 106 means by definition that there are 106 males per 100 females. So sex ratio 106 doesn't confirm with the traditional mathematical definition of ratio. Also if sex ratio is given as 1.06 then one should conclude that  there are 1.06 males per 1 female which is equivalent to the previous statement of 106:100. Now maternal mortality ratio (MMR) is defined as annual number of maternal deaths due to pregnancy related causes per 100,000 live births. But maternal mortality rate (MMR)is also number of maternal deaths per 100, 000 live births. Similarly Crude death rate (CDR) is number of deaths per 1000 people in the population and Infant mortality rates (IMR) number of deaths of infants per 1000 live births.

Intuitive approach is the best possible option for finding a way through this ambiguity. We should not be carried away by these numbers and loose our way in this ambiguity of definition. We should rather link these mathematical figures with country specific scenario. This way we will not loose track of the main objective, which is  getting a clear picture of development status of that country.

.........and one more tip in case of frequent events  like birth and death it is per 1000 and in case of less frequent  events like cause specific death rate it is expressed per 100,000.