We can use per capita birth (b) and death (d) rates to calculate a new parameter "r." This parameter is essentially a per capita growth rate, and is calculated as:
"r" is variously referred to as the "rate of natural increase" or the "per capita rate of increase" of the population, or as the "rate of net reproduction per individual." It is essentially the probability that any individual in the population will give birth during the time interval (usually year), discounted for their probability of dying.
It is analogous to net income, which is gross income minus taxes. Here the "income" is births and the "taxes" are deaths.
Expressing the rate of net reproduction (or rate of natural increase) on a per capita basis like this is useful for the same reason that it is useful to express birth and death rates on a per capita basis; it facilitates intercomparisons among populations of differing sizes. It is independent of the population's actual size; a population with a small "r" is increasing at a slower per capita rate than a population with a larger "r."
(If you're curious, click on "Current " for notes on the current human population situation, and information about "r" for various regions of the world.)
Check yourself #3: What is "r" for the U.S. in 2008, using the per capita birth and death rates that you calculated in the section on per capita birth and death rates? (Click on "Answers " to check yourself.)
(Click ">>" box to move to the next section (on a new way to calculate "G") now; "<<" box to move to preceding section, "Contents" box to return to master directory for the BI301 web site. "Navigate " gives general reminders on how to do that.)