# Parity

This article describes a measure that can be evaluated for a particular individual person, household, or family. It is not a statistical aggregate, though it may be statistically aggregated to provide average measures and distributions.
The measure may vary with time, i.e., its value may be different at different points in time.
The measure is not subjective, as the measurement of its value is not based on people's beliefs, opinions, and/or values. Note that the actual value itself is influenced by people's decisions, and hence by their subjective preferences, but the measurement of these values is not.

## Definition

The parity of a female at a given point in time is defined as the number of live births of babies to the female so far. The parity of a female at a given point in time must be a nonnegative integer and is typically not more than 20. The parity may change with time. It cannot decrease -- it either stays the same or goes up in increments of one or higher (higher increments occur in the rare case that the female gives birth to twins, triplets, or their higher-order counterparts).

Parity values are often used to subdivide the female population into subpopulations of interest. For instance, we might study zero-parity females and contrast them with one-parity females and two-parity females.

## Related notions

• Expected fertility for a woman at a given point in time is the total number of children the woman expects to have in her lifetime. We have the following relation between parity and expected fertility at any point in time:

Expected fertility $\ge$ Parity

• Completed fertility is the parity of a female who has finished her childbearing years and thus will have no more children. Note that the value of completed fertility is a constant, i.e., it does not change with time.