# Stocks and Flows, Individuals and Events

In the real world, populations grow or decline through a process of entries and exits. There are several ways through which individuals enter or exit populations. In the most general case (that of the population of a country), there are three categories of entries and exits: births, deaths and migrations. In fact, the size of a country's population at any point in time (for example, 1/1/2010) can be expressed very simply as 1) the population a year earlier (i.e., 1/1/2009) plus 2) the number of births that have occurred in the population in the intervening year, minus 3) the number of deaths that have occurred in the population, plus 4) the number of immigrants who have moved into the population, minus 5) the number of emigrants who have moved out of the population. This is what demographers refer to as the **"balancing equation"**.

(1) Starting population (July 1, 2001) | 285,317,559 |

+ (2) Births (7/1/2001 - 6/30/2002) | + 4,047,642 |

- (3) Deaths (7/1/2001 - 6/30/2002) | - 2,445,837 |

+ (4) Immigrants (7/1/2001 - 6/30/2002) | + 1,664,334 |

- (5) Emigrants (7/1/2001 - 6/30/2002) | - 215,000 |

= Ending Population (July 1, 2002) | = 288,368,698 |

The difference between births and deaths is called a population's **"natural increase"**, whereas the difference between the number of immigrants and the number of emigrants is the **"net migration"**.

The balancing equation applies to any population, at any point in time. It relates two *stocks*, i.e., two counts of individual members of a population taken at two points in time, by the *flows* that affect them. A *flow* is the number of events that have occurred within a population between two time points.