The median age is the age that divides the population into two equal-size groups. Half of the population is older than the median age and half is younger. Median age is based on a standard distribution of the population by single years of age and is shown to the nearest tenth of a year. (See the sections on "Standard Distributions" and "Medians" under "Derived Measures.")

The age dependency ratio is derived by dividing the combined under-18 and 65-and-over populations by the 18-to-64 population and multiplying by 100.

The old-age dependency ratio is derived by dividing the population 65 years and over by the 18-to-64 population and multiplying by 100.

The child dependency ratio is derived by dividing the population under 18 years by the 18-to-64 population, and multiplying by 100.

Caution should be taken when comparing population in age groups across time. The entire population continually ages into older age groups over time and babies fill in the youngest age group. Therefore, the population of a certain age is made up of a completely different group of people in 2000 and 2007. Since populations occasionally experience booms/increases and busts/decreases in births, deaths, or migration (for example, the postwar Baby Boom from 1946-1964), one should not necessarily expect that the population in an age group in Census 2000 should be similar in size or proportion to the population in the same age group in the 2007 ACS. For example, Baby Boomers were age 36 to 54 in Census 2000 while they were age 44 to 62 in the 2007 ACS. Therefore, the age group 55 to 59 would show a considerable increase in population when comparing Census 2000 data with the 2007 ACS data.
Beginning in 2006, the population in group quarters (GQ) is included in the ACS. Some types of GQ populations have age distributions that are very different from the household population. The inclusion of the GQ population could therefore have a noticeable impact on the age distribution. This is particularly true for areas with a substantial GQ population.

The 1996-2002 American Community Survey question asked for month, day, and year of birth before age. Since 2003, the American Community Survey question asked for age, followed by month, day, and year of birth. In 2007, an additional instruction was provided with the age and date of birth question on the American Community Survey questionnaire to report babies as age 0 when the child was less than 1 year old. The addition of this instruction occurred after 2005 National Census Test results indicated increased accuracy of age reporting for babies less than one year old.

For data products, a "civilian veteran" is a person 18 years old or over who has served (even for a short time), but is not now serving, on active duty in the U.S. Army, Navy, Air Force, Marine Corps, or the Coast Guard, or who served in the U.S. Merchant Marine during World War II. People who served in the National Guard or military Reserves are classified as veterans only if they were ever called or ordered to active duty, not counting the 4-6 months for initial training or yearly summer camps. All other civilians 18 years old and over are classified as nonveterans.

People who indicated that they had served on active duty or were now on active duty were asked to indicate the period or periods in which they served. The responses to the question about period of service were edited for consistency and reasonableness. The edit eliminated inconsistencies between reported period(s) of service and age of the person; it also removed reported combinations of periods containing unreasonable gaps (for example, it did not accept a response that indicated that the person had served in World War II and in the Vietnam era, but not in the Korean conflict).

For the 1996-1998 American Community Survey, people who indicated that they had served on active duty or were now on active duty were asked to report the total number of years and months of active-duty service.
Since 1999, people who indicated that they had served on active duty or were now on active duty were asked to report whether the total number of years of active-duty service was less than 2 years or 2 years or more. The question asked for accumulated service, which is not necessarily the same as continuous service.

In determining the poverty status of families and unrelated individuals, the Census Bureau uses thresholds (income cutoffs) arranged in a two-dimensional matrix. The matrix consists of family size (from one person to nine or more people) cross-classified by presence and number of family members under 18 years old (from no children present to eight or more children present). Unrelated individuals and two-person families are further differentiated by age of reference person (RP) (under 65 years old and 65 years old and over).
To determine a person's poverty status, one compares the persons total family income in the last 12 months with the poverty threshold appropriate for that person's family size and composition (see example below). If the total income of that person's family is less than the threshold appropriate for that family, then the person is considered "below the poverty level," together with every member of his or her family. If a person is not living with anyone related by birth, marriage, or adoption, then the person's own income is compared with his or her poverty threshold. The total number of people below the poverty level is the sum of people in families and the number of unrelated individuals with incomes in the last 12 months below the poverty threshold.
Since ACS is a continuous survey, people respond throughout the year. Because the income questions specify a period covering the last 12 months, the appropriate poverty thresholds are determined by multiplying the base-year poverty thresholds (1982) by the average of the monthly inflation factors for the 12 months preceding the data collection. See the table below titled "Poverty Thresholds in 1982, by Size of Family and Number of Related Children Under 18 Years (Dollars)," for appropriate base thresholds. See the table "The 2007 Poverty Factors" for the appropriate adjustment based on interview month.
For example, consider a family of three with one child under 18 years of age, interviewed in July 2007 and reporting a total family income of $14,000 for the last 12 months (July 2006 to June 2007). The base year (1982) threshold for such a family is $7,765, while the average of the 12 inflation factors is 2.19359. Multiplying $7,765 by 2.19359 determines the appropriate poverty threshold for this family type, which is $17,033. Comparing the familys income of $14,000 with the poverty threshold shows that the family and all people in the family are considered to have been in poverty. The only difference for determining poverty status for unrelated individuals is that the person's individual total income is compared with the threshold rather than the familys income.
For example, consider a family of three with one child under 18 years of age, interviewed in July 2007 and reporting a total family income of $14,000 for the last 12 months (July 2006 to June 2007). The base year (1982) threshold for such a family is $7,765, while the average of the 12 inflation factors is 2.19359. Multiplying $7,765 by 2.19359 determines the appropriate poverty threshold for this family type, which is $17,033. Comparing the familys income of $14,000 with the poverty threshold shows that the family and all people in the family are considered to have been in poverty. The only difference for determining poverty status for unrelated individuals is that the person's individual total income is compared with the threshold rather than the familys income.
The 2007 Poverty Factors:

Interview Month	Poverty Factors
January	2.14841
February	2.15589
March	2.16297
April	2.17003
May	2.17705
June	2.18455
July	2.19359
August	2.20366
September	2.21330
October	2.22219
November	2.22879
December	2.23073

Poverty Thresholds in 1982, by Size of Family and Number of Related Children Under 18 Years Old (Dollars)

Size of family unit	Related children under 18 years
	None	One	Two	Three	Four	Five	Six	Seven	Eight or more
One person (unrelated individual)
Under 65 years	5,019
65 years and over	4,626
Two persons
Householder under 65 years	6,459	6,649
Householder 65 years and over	5,831	6,624
Three persons	7,546	7,765	7,772
Four persons	9,950	10,112	9,783	9,817
Five persons	11,999	12,173	11,801	11,512	11,336
Six persons	13,801	13,855	13,570	13,296	12,890	12,649
Seven persons	15,879	15,979	15,637	15,399	14,955	14,437	13,869
Eight persons or more	17,760	17,917	17,594	17,312	16,911	16,403	15,872	15,738
Nine persons or more	21,364	21,468	21,183	20,943	20,549	20,008	19,517	19,397	18,649

Poverty status was determined for all people except institutionalized people, people in military group quarters, people in college dormitories, and unrelated individuals under 15 years old. These groups were excluded from the numerator and denominator when calculating poverty rates.

For various reasons, the official poverty definition does not satisfy all the needs of data users. Therefore, some of the data reflect the number of people below different percentages of the poverty thresholds. These specified poverty levels are obtained by multiplying the official thresholds by the appropriate factor. Using the previous example cited (a family of three with one related child under 18 years responding in July 2007), the dollar value of 125 percent of the poverty threshold was $ 21,291 ($ 17,033x 1.25).

Income deficit represents the difference between the total income in the last 12 months of families and unrelated individuals below the poverty level and their respective poverty thresholds. In computing the income deficit, families reporting a net income loss are assigned zero dollars and for such cases the deficit is equal to the poverty threshold. This measure provides an estimate of the amount, which would be required to raise the incomes of all poor families and unrelated individuals to their respective poverty thresholds. The income deficit is thus a measure of the degree of the impoverishment of a family or unrelated individual. However, please use caution when comparing the average deficits of families with different characteristics. Apparent differences in average income deficits may, to some extent, be a function of differences in family size.

Aggregate income deficit refers only to those families or unrelated individuals who are classified as below the poverty level. It is defined as the group (e.g., type of family) sum total of differences between the appropriate threshold and total family income or total personal income. Aggregate income deficit is subject to rounding, which means that all cells in a matrix are rounded to the nearest hundred dollars. (For more information, see "Aggregate" under "Derived Measures.")

Mean income deficit represents the amount obtained by dividing the total income deficit for a group below the poverty level by the number of families (or unrelated individuals) in that group. (The aggregate used to calculate mean income deficit is rounded. For more information, see "Aggregate Income Deficit.") As mentioned above, please use caution when comparing mean income deficits of families with different characteristics, as apparent differences may, to some extent, be a function of differences in family size. Mean income deficit is rounded to the nearest whole dollar. (For more information on means, see "Derived Measures.")

When the original poverty definition was developed in 1964 by the Social Security Administration (SSA), it focused on family food consumption. The U.S. Department of Agriculture (USDA) used its data about the nutritional needs of children and adults to construct food plans for families. Within each food plan, dollar amounts varied according to the total number of people in the family and the family's composition, that is, the number of children within each family. The cheapest of these plans, the Economy Food Plan, was designed to address the dietary needs of families on an austere budget.
Since the USDAs 1955 Food Consumption Survey showed that families of three or more people across all income levels spent roughly one-third of their income on food, the SSA multiplied the cost of the Economy Food Plan by three to obtain dollar figures for total family income. These dollar figures, with some adjustments, later became the official poverty thresholds. Since the Economy Food Plan budgets varied by family size and composition, so too did the poverty thresholds. For two-person families, the thresholds were adjusted by slightly higher factors because those households had higher fixed costs. Thresholds for unrelated individuals were calculated as a fixed proportion of the corresponding thresholds for two-person families. The poverty thresholds are revised annually to allow for changes in the cost of living as reflected in the Consumer Price Index for All Urban Consumers (CPI-U). The poverty thresholds are the same for all parts of the country; they are not adjusted for regional, state, or local variations in the cost of living .

Since poverty is defined at the family level and not the household level, the poverty status of the household is determined by the poverty status of the householder. Households are classified as poor when the total income of the householder's family in the last 12 months is below the appropriate poverty threshold. (For nonfamily householders, their own income is compared with the appropriate threshold.) The income of people living in the household who are unrelated to the householder is not considered when determining the poverty status of a household, nor does their presence affect the family size in determining the appropriate threshold. The poverty thresholds vary depending upon three criteria: size of family, number of children, and, for one- and two- person families, age of the householder.

Beginning in 2006, the population in group quarters (GQ) is included in the ACS. The universe for most disability data tabulations is the civilian noninstitutionalized population. Some types of GQ populations have disability distributions that are different from the household population. The inclusion of the noninstitutionalized GQ population could therefore have a noticeable impact on the disability distribution. This is particularly true for areas with a substantial noninstitutionalized GQ population. For example, the number of people with a disability may increase in areas having a substantial group home population. In areas having a substantial college dormitory population, the percentage of people with a disability may decrease because the base of the percentage, which now includes the population in college dormitories, is larger.

The data on sensory and physical limitations were derived from answers to Questions 15a and 15b, which were asked of people 5 years old and over. Questions 15a and 15b asked respondents if they had any of the following two long-lasting conditions: "Blindness, deafness, severe vision or hearing impairment," or "A condition that substantially limits one or more basic physical activities such as walking, climbing stairs, reaching, lifting, or carrying." Respondents were instructed to mark "yes" or "no" for each long-lasting condition. Question 15a is labeled as "Sensory disability" and Question 15b as "Physical disability" for some of the disability data products such as the ACS Detailed Tables.

The data on self-care limitations were derived from answers to Question 16b, which was asked of people 5 years and over. The question asked respondents if they had a physical, mental, or emotional condition lasting 6 months or more that made it difficult "dressing, bathing, or getting around inside the home." Respondents were instructed to mark "yes" or "no." Question 16b is labeled as "Self-Care Disability" for some disability data products such as the ACS Detailed Tables.