Final answer:
The crude birthrate is the measure of births per 1000 individuals in a given time, used to calculate the population growth rate, with 12 per 1,000 being the rate for the U.S. It's an essential factor, along with the death rate, to determine the net population change and understand demographic trends and health indicators like the infant mortality rate.
Step-by-step explanation:
The crude birthrate is expressed as the births per 1000 individuals per unit of time. This measure essentially quantifies the number of live births in a given year for every 1,000 people alive in the society at that time. It is an important factor considered when calculating the net population growth rate. If the birth rate exceeds the death rate, the population grows, and if it's the reverse, the population decreases. The U.S., for example, has a birth rate of about 12 people per 1,000 per year and a death rate of 8.1 per 1,000 per year, leading to a net growth rate of approximately 0.4% per year.
An interesting case is presented when analyzing countries in the early stages of demographic transition, where the death rate might be significantly lowered but the birth rate remains relatively high. For instance, a death rate of 15 per 1,000 and a birth rate of 35 per 1,000 results in a net increase of people added to the population each year per 1,000 people, and helps understand the rate of growth in percentage terms.
Knowing these rates, including infant mortality rate which measures the annual number of infant deaths per 1,000 live births, provides insights into the health and growth trends of a population.