South Dakota Population Pyramids
By County
By
Trevor Brooks, Michael McCurry,
Marcey Moss, David P. Olson, and Donald E. Arwood
2001
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South Dakota Population Pyramids
By |
INTRODUCTION
Age and sex structure of a population plays a critical role in determining the needs and lifestyles of that population. One community, for example, may have a high number of children and youth, indicators of the need for teachers, schools and youth recreational services. Another community may have a high proportion of elderly, indication the need for medical, nutritional, and home care services. People of different ages or gender also may have different consumer preferences, which can influence business trends.
When business and community leaders know the age and sex structure of their community, they can us that information in planning the use of resources toward meeting particular needs in education, housing, recreation, community infra structure, and medical and social services.
No two communities are likely to have identical age and sex structures. However, there are general patterns that are helpful for identifying and understanding potential population needs. Population pyramids and indices are useful for illustrating these patterns.
POPULATION PYRAMIDS
A population pyramid is a graphic representation of a population’s (state, county, town, etc.) age and sex structure. It is a type of bar chart. The proportions of males and females for each age group (cohort) are shown as a series of horizontal bars stacked on top of each other.
With the exception of the 75+ age group, each bar represents a five-year cohort. Bars representing the proportion of males in each age group are on the left hand side of the graph; bars representing females for each age group are on the right. A frequency table that provides additional details also accompanies each population pyramid.
A population pyramid tells a great deal about the population in question. South Dakota’s 2000 Census population falls into five general structural patterns.
An EXPANSIVE pyramid (larger bars at the bottom of the pyramid) reveals that a large proportion of the population is young. Native American counties are good examples of expansive populations. Each birth cohort is larger than the one that preceded it. This indicates that the population is growing. These populations have needs and preferences associated with younger populations (also see the following discussion of youth dependency ratios).
Brookings and Clay counties are good examples of CONSTRICTIVE pyramids. The shape is constricted at the bottom. Each new birth cohort is smaller than the preceding one so the growth rate is declining. Brookings and Clay counties have constricted populations under the age of 15 because they have large populations of college-aged residents.
A NEAR STATIONARY pyramid is roughly rectangular in shape. It has approximately equal proportions of people in all age groups with a narrowing of the pyramid at the middle-age cohorts. It indicates that the population is neither increasing nor decreasing. The 2000 pyramids for Minnehaha and Pennington counties are good examples of this type of population structure. In South Dakota, this pattern is associated with urban populations that have diverse economies.
High Migration is another type of pyramid pattern. Many farm-dependent counties have high out-migration of young adults (ages 18-24). This phenomenon is clearly revealed in their population pyramids. These pyramids often appear to be similar to the stationary type except that the bars for age categories with high out migration rates are shorter than would otherwise be expected. This type usually has relatively larger proportions of people aged sixty-five and over (also see the discussion of the aged-dependency ratio and the index of aging).
Brown and Hughes county pyramids are examples of the BABY BOOM pattern. In 2000, these pyramids have two distinct bulges. One represents the baby boom (ages 35-54), and the other represents the baby boom echo (ages 10-19). The smaller cohorts between them are called the “baby bust” (ages 20-34). The needs and preferences of these populations tend to be associated with families and young children.
POPULATION INDICES
Population indices provide further detail by summarizing the relationships between various segments of the population. Eight indices have been included: median age, sex ratio, general dependency ratio, youth dependency ratio, aged dependency ratio, young adult ratio, index of aging, and fertility ratio. Each of these indices is described below, along with the mathematical formulas for calculating them
1. Median Age. The median age is the most commonly used measure of the average age of a population. In 2000, South Dakota’s median age was 35.6 years. This means that half of the population was younger than 35.6 and half was older than 35.6.
Using this definition, McPherson County had the oldest population with a median age of 47.6--half were older than 47.6 and half were younger than 47.6. Shannon County had the youngest population with a median age of 20.6.
2. Sex Ratio. The sex ratio identifies how many males there are per 100 females. In 2000, Bon Homme County had the highest sex ratio, with 123.4 males per 100 females. Brule County had the lowest, with 93.16 males per 100 females. The sex ratio is the simplest measure of the population structure.
Number of males
Sex ratio = x 100
Number of females
3. General Dependency Ratio (GDR). This ratio (also known as the total dependency ratio) describes the proportion of the population in age group that tend to be economically dependent, relative to the proportions of the population that tend to be economically productive.
Although the upper and lower age boundaries are somewhat arbitrary, the segment of the population, which is said to be productive, is the group aged 15 through 64. The dependent population are those under the age of 15 and 6 years and older.
South Dakota’s 2000 GDR was 56.71; this means that there were 56.71 persons in the dependent age group per 100 people in the economically productive age group. Counties with high general dependency ratios have population structures similar to the “expansive” pyramid pattern (a very young population) or the “high migration” pattern (proportionately fewer people age 15 to 64).
Persons Persons
under age 15 + age 65
and over
GDR = X 100
Number of persons age 15 to 64
4. Youth Dependency Ratio (YDR). This measure identifies the number of children (age 0 t0 15) per 100 productive people in the population (persons age 15 to 64).
In 2000, South Dakota’s YDR was 34.26 the means that there were 34.26 dependent youths per 100 persons in the productive age group. Shannon County with 65.37 had the highest youth dependency ratio among all counties in the state. Other counties with high youth dependency ratios were Dewey, Todd, and Ziebach. The pyramids for these counties are of the “expansive” type.
Number of persons
under age 15
YDR = X 100
Number of persons age 15 to 64
5. Aged Dependency Ratio (ADR). This dependency measure identifies the number of elderly (age 65 and older) per 100 productive people in the population (persons age 15 to 64).
In 2000, South Dakota’s aged dependency ratio was 22.45. McPherson County’s ADR (56.51) was more than twice that. Besides having a large proportion of elderly, most counties with high aged dependency ratios also show a high out-migration of young adults and have pyramids of the high migration type.
Number of persons
age 65 and over
ADR = U X 100
Number of persons age 15 to 64
6. Young Adult Ratio (YAR). The young adult ratio identifies the number of potential parents with young children in the population. This statistic is not a dependency measure; instead, it indicates the population’s potential for fertility. It is calculated by dividing the number of persons age 15 through 34 by the number of persons younger and older than this group.
South Dakota’s 2000 young adult ratio was 37.60. The YAR’s of Clay County (93.1) and Brookings County (74.09) were much larger then the state average. Shannon and Minnehaha also had high young adult ratios.
Number of persons
age 15 to 34
YAR = X 100
Number of persons Number of persons
under age 15 + age 35 and over
7. Index of Aging. This index is a measure of the age structure of the population. It compares the old to the young of a population by dividing the number of elderly (age 65+) by the number of children (under 15 years old) in the population. A score of 100 means that there are 100 elderly for every 100 persons under age fifteen. Scores of 100 or more are very high.
In 2000, South Dakota had 20 counties with index of aging scores that exceeded 100. The four counties with the largest index were: McPherson (163.62), Jerauld (160.22), Potter (138.68), and Gregory (131.67).
Number of persons age 65+
Index of aging = X 100
Number of persons under age 15
8. Fertility Ratio (FR). The fertility ratio, often called the child-woman ratio, compares the number of children in the population under age five to the number of women in their childbearing years. Although it is not a measure of the actual number of births in a population, the fertility ratio is a good indicator of fertility behavior in the population. It is usually used when vital statistics data are not available.
For example, at the height of the baby boom in 1960, the fertility ratio for South Dakota was 670.44. This means that there were 670.44 children under age 5 for every 1,000 women age 15 through 44. By 1970, the baby boom had ended and the fertility ratio had dropped to 424.47.
Number of persons under age 5
FR = X 100
Number of women age 15 to 44
Summary
This report is an update of one of the most popular publications of SDSU’s Census Data Center. It provides population pyramids, frequency tables, and descriptive indices for each county in South Dakota. The statistics contained in this publication are based on the decennial censuses. The source for 2000 data is the “2000 Census of Population and Housing. The source for 1990 data is the “1990 Census of Population and Housing, Summary Tape File 1A, Vol. 1 (CD-ROM),” U.S. Department of Commerce. The data for 1980 are from “United States Population, 1980, South Dakota: General Population Characteristics. PC80-1-B43,” U.S. Department of Commerce. Data for pyramids prior to 1980 are from “South Dakota Population, Age and Sex Structure, 1970-1980.” Update Series C229, No.26, Department of Sociology, South Dakota State University, Brookings, South Dakota.