The expectation of life (cont.)
The stationary population (cont.)
If this diagram is rotated it begins to look remarkably like one half of a population pyramid, with the age groups represented by Lx values.
Figure 2: A rotated graph of Figure 1.
And indeed it can be conceptualised as a theoretical population - the stationary population.
What is this population and why is it stationary? It is the population structure that would occur if there were radix births a year and the mortality rates of the life table continued over a long period of time. This emphasises an important demographic principle - population structure is entirely the result of fertility and mortality rates applied over a long period of time. Put another way, if mortality and/or fertility rates change the population structure starts to change. If the new rates continue for a long period then a new structure develops - irrespective of the previous structure.
This stationary population is a hypothetical application of this principle. It is "stationary" because there is no growth. Further demographic theory allows us to consider structures where there is population growth - this is the more advanced topic of stable population theory, but understanding the stationary population is an introduction to this.