title: Parity progression ratios
text: # Parity progression ratios
  
  A parity progression ratios (PPR) is a measure commonly used in demography to study fertility. The PPR is simply the proportion of women with a certain number of children who go on to have another child.
  
  Calculating the PPR, also known as $a_{x}$, can be achieved by using the following formula:
  
  $a_{x}={\text{(women with at least }}x+1{\text{ children ever born) }}/{\text{ (women with at least }}x{\text{ children ever born)}}$
  
  In more developed countries where two-child families are seen as the norm $a_{2}$ (or the proportion of women with two children who go on to have a third) is of critical importance in determining overall fertility levels.
  
  $a_{0}$ is simply the proportion of women who become mothers.
  
  