It is often useful to express population parameters such as birth and death rates on a per capita (per person) basis. This helps us to compare what is going on in populations of differing sizes. It standardizes across different population sizes by expressing these parameters on a per person basis.
For example, if I tell you that there were five births in a population in a year, that doesn't tell you much about what's really going on in that population. Is five a lot or a little? The answer depends on the number already in that population.
To determine per capita birth or death rates, you simply divide the absolute number of births ("B") or deaths ("D") by the number in the population ("N") at the midpoint of the time interval (usually year). By convention, for human demographics, we use the total number ("N") of people, regardless of age or sex.
We will use lower case "b" and "d" to indicate per capita birth and death rates, respectively.
Thus, b = B/N and d = D/N.
Check yourself #1: Use the following data to calculate "b" and "d" for the U.S. in 2012. (Click on "Answers " here to jump to the answers, after you have done the calculation.)
US Population size in 2012 (N) = approximately 314 million
Number of births in the US in 2012 (B) = approximately 4.1 million
Number of deaths in the US in 2012 (D) = approximately 2.5 million
What is the per capita birth rate ("b") for the US in 2012?
What is the per capita death rate ("d") for the US in 2012?
Incidentally, net immigration [(legal + estimated illegel + refugees) - emigration] is estimated to add about 628,000 people to the US per year as of 2012 according to the Population Reference Bureau. Compare that number to the number of births in the US to get some idea of the relative influence of births versus immigration on the US population. (We'll explore more about effects of immigration on the US population when we discuss the current status of the human population; click on Current to move to that topic now, if you wish.)
These per capita birth and death rates for humans can also be viewed as the probability that any one person will give birth or die in a year (across all sexes and age classes). (Remember, probabilities range from 0 to 1. Something with probability of 0 basically never happens, while with probability 1, something is certain to happen. Probabilities can also be derived from percentages, by dividing the percentage by 100.)
(Click ">>" box at the bottom to move to the next section (on "crude" birth and death rates) now; "<<" box to move to preceding section, "Contents" box to return to the master directory for the BI301 web site.)