Population is constantly changing due to births, deaths, aging, and the migration of people with different social and cultural characteristics. Monitoring the growth and loss of population and the changes in the characteristics of the population is a major focus of demographic research.
Reporting change, however, is not just a matter of reporting the absolute change in the number of persons. It is often more important to know how many persons were gained or lost relative to the total number that were in an area initially. This is usually expressed as a percentage increase or decrease. Percentage change often gives a very different impression than is obtained from comparing absolute numbers.
Very commonly smaller places experience the greatest percentages of change so that population growth or decline often has a relatively greater impact on smaller places. However, sometimes places are so small that percentages become misleading. For example, the highest percentage of American Indians in Los Angeles County in 1980 by far was in a tract with 12.5% of its population American Indian. A closer examination of this data revealed, however, that there were only eight people in the tract, one of whom reported a race of American Indian. Thus, it is usually advisable to also report the absolute number of persons when reporting percentages. Of course, politicians, the media, business people, and consultants will use whichever figure best suits their needs when discussing change.
A. Describing Population Change
The table below illustrates several expressions of population change between 1980 and 1990 for California counties. If one were to rank the counties by the gain in the numbers of Mexican-origin persons (first column), Los Angeles County gained nearly four times more Mexican-origin people than the next county. Los Angeles County claimed nearly a third of all Mexican-origin persons who came to the State of California. Nearly half of the Mexican-origin population increase in the entire United States occurred in California.
The second column ranks the counties by percentage increase in the number of Mexican-origin persons. Here a very different set of counties emerges. When ordered by number, urban counties show the most growth; but when ordered by percentage increase, mostly rural counties in the Sierra foothills, the eastern San Joaquin Valley and the northern mountain areas show the greatest gain. These counties are not very populous, but they underwent the greatest percentage increase in Mexican-origin persons relative to the number of Mexican-origin persons that were there in 1980.
The third column shows the difference in the percentage of the total population that is Mexican-origin between 1980 and 1990. Colusa County, for example, had an increase of 12.8 percentage points in the total population that is Mexican origin. Those counties showing the greatest increase in the percentage of their residents who are Mexican origin are mostly agricultural counties. Mexican-origin persons have had the greatest increase in the percentage of the total population there. Amador, Mono, El Dorado and other counties in the second column are not in the third column because their percentages of Mexican-origin people are so low in their population.
Table 9. Greatest Mexican-origin Population Changes in California Counties
1980 - 1990
Area Change in
Area Percent Change Area 1990 %
1.6 California 2,481,530 California 68.2 California 5.2 Los Angeles 876,226 Amador 320.1 Colusa 12.8 Orange 242,346 Mono 205.3 Imperial 9.4 San Diego 210,778 El Dorado 177.7 Tulare 8.3 San Bernardino 179,345 Tehama 169.1 Glenn 8.2 Riverside 159,812 Del Norte 154.4 Orange 7.7 Santa Clara 77,011 Lassen 148.9 Santa Barbara 7.7 Fresno 76,104 Nevada 146.9 Madera 7.4 Ventura 57,001 Mendocino 146.3 Monterey 7.3 Kern 55,754 Riverside 144.9 Kings 7.1 Tulare 44,431 Modoc 139.7 Merced 6.8
B. The Effect of Migration and Residential Mobility on Population Change
The Demographic EquationPopulation change always occurs as the result of births, deaths, and net migration. The basic equation showing the interrelationship of these components with total population change over a specific time is referred to as the Demographic Equation.
Pop Change = Births - Deaths + In-migrants - Out-migrantsThe excess of births over deaths is called natural increase and the difference between in-migration and out-migration is called net migration. Distinguishing between natural increase and net migration provides important information on the forces behind population changes in any state or county.
The Demographic Equation provides a mechanism for estimating population between the decennial censuses. A number of agencies such as the California Department of Finance estimate population annually between the censuses. One method the Dept. of Finance uses is the Drivers' License Address Change method which takes into account births, deaths, and other data distinctive to three age groupings. For the youngest age group, the Department of Finance uses changes in school enrollment by grades and for the people age 65 and older the agency uses changes in Medicare enrollment. Estimating the population ages 15 to 64 is done by measuring changes in drivers license addresses that have been adjusted with tax return data and immigration data. Substituting actual California state values for the period of 1990 to 1997 into the Demographic Equation:
|1990 Population||Births||Deaths||Net Migration|
The estimated 1997 population of California is 32,957,000. An interesting sidelight in this Department of Finance data are the values reported for net immigration (1,750,114) and net domestic migration (-1,230,892). This indicates that California had a large outmigration to other states during this seven-year period, but this was more than balanced demographically by the large numbers arriving from other countries. The table shows that natural increase accounted for almost 2.5 million of the total population in California during these seven years. In contrast, net migration accounted for only 17 percent of California's growth. Thus, it is correct to say that 83 percent of the state's population growth between 1990 and 1997 was due to natural increase.
Births and DeathsThe contribution of births or deaths to the population change is often expressed as a rate per 1000 persons. The Crude Birth Rate of any area (and the Crude Death Rate) is the number of births (or deaths) in a year multiplied by 1000 and divided by the total population of that area. Often the mid-year population is estimated for the denominator by averaging the beginning and ending populations. For the California data above we can compute an average Crude Birth Rate and Crude Death Rate over the seven year period:
4,035,266 births / 7 * 1000 / ((32,957,000 + 29,944,000) / 2) = 18.3 births /1000 persons per yearThe birth rate may be further modified to take into account the fact that usually only women between ages 15 and 44 bear children. Using only this number as the denominator yields the General Fertility Rate within a population.
1,541,488 deaths / 7 * 1000 / ((32,957,000 + 29,944,000) / 2) = 7.0 deaths / 1000 persons per year
These rates, however, do not take into account differences in the age structure of populations. This is a minor problem in interpreting Crude Birth Rates, but differences in age structure between countries has a very large effect on Crude Death Rates. The problem is overcome by calculating age-specific fertility rates (which can be combined to produce a Total Fertility Rate) and age-specific mortality rates (such as the Infant Mortality Rate and Life Expectancy).
Migration and Local Residential MobilityGeographical mobility can include travel and seasonal circulations such as those of "snowbirds" or farm workers. However, most research focuses on residential moves that result in a change of permanent address. People move for a variety of reasons including the desire for better jobs, schools, and housing, being closer to relatives, or for living in a more attractive environment, perhaps near recreation.
There are two types of moves: migration and local residential mobility. These types differ according to the distance of the moves. Moves which are far enough to disrupt one's employment and social networks constitute migrations. Shorter moves, often to a different house in the same part of a city, are considered local residential mobility. Local mobility shifts are typically driven by changing housing needs. In the United States, researchers usually deal with this distinction by considering a change of address or residence beyond the current county of residence to be a migration whereas movement within a county is considered local residential mobility.
A problem with census data is that it measures movement only during discrete time periods such as the last five years. People indicate only their residence five years earlier even though they may have made several moves during that period.
Like birth and death, migration may also be expressed as a rate. California, for example, had a net migration rate during the 1990-1997 period of 17.3 persons per thousand.
519,222 * 1000 / 29,944,000 = 17.3 persons per 1000The Census Bureau creates several tabulations which are useful for exploring dimensions of migration. Table 10 (derived from STF3a, P37) indicates citizenship for persons in California. One would expect that over time immigrants would be assimilated into the larger population through naturalization. About 22% of the State's population in 1990 was born outside the United States. These are, by definition, immigrants. Of these, about two thirds remained non-citizens as of 1990.
(If the direction of the net migration had been out of California, the rate would have been negative.)
Table 10. Citizenship, California, 1990 U.S. Born Foreign Born Naturalized Foreign Born
Not a Citizen
Persons 23,301,196 2,017,610 4,441,215 Percent of Population 78.3 6.8 15.0
Table 11 presents the region of birth for the California population (STF3a, P42). This dimension of migration indicates lifetime shifts. Many of the people lived in several states before coming to California, but they did eventually live here in 1990. Less than 50% of the state's population was born in California. Over 19% were born in the South or Midwest while over 23% were born outside the United States.
|Percent of Population||46.4||6.1||10.6|
|Percent of Population||8.5||5.5||23.0|
Table 12, tabulated for persons aged 5 and older, presents shifts that have occurred between a person's residence in 1985 and 1990 (STF3a, P43). Over 44% of California's population aged 5 and older were living in the same house both years. Over 75% were living in the same county, and 87% were living in the same state five years earlier.
|Living in Same House||Living in Different House in Same County||Living in Same State in Different County||Living in NE U.S.|
|Persons Age 5+ Years||12,146,574||8,525,870||3,237,662||308,829|
|Percent of Population||44.4||31.1||11.8||1.1|
|Living in Midwest U.S.||Living in South U.S.||Living in West U.S.||Living Outside U.S.|
|Persons Age 5+ Years||423,473||589,651||652,880||1,498,608|
|Percent of Population||1.5||2.2||2.4||5.4|
Although less than six percent of the state's 1990 population immigrated from another county during the 1985-1990 years, this group is often concentrated in certain cities and neighborhoods. These most recent arrivals will be experiencing the stresses of adaptation and acculturation most intensively, and thie places where they are concentrated are the ones most strongly impacted by immigration.
The U.S. Census has other tables that provide further information on migration and mobility of the population. However, it is not possible here to probe all the possible dimensions of migration and mobility. In STF3a, P36 tabulates year of entry for foreign-born persons, P44 provides 1985 place of residence by metropolitan area, and H28 indicates the year a householder moved into a housing unit. For even more detail, STF4 provides ethnic and age categories for migration and mobility data.