Spatial Thinking in Planning Practice: An Introduction to GIS

36 As you can see, the e#ect of the two di#erent classi!cation schemes on the appearance of the two choropleth maps above is dramatic. "e quantiles scheme is o$en preferred because it prevents the clumping of observations into a few categories shown in the equal intervals map. Conversely, the equal interval map reveals two outlier counties that are obscured in the quantiles map. Due to the potentially extreme di#erences in visual appearance, it is o$en useful to compare the maps produced by several di#erent map classi!cations. Patterns that persist through changes in classi!cation schemes are likely to be more conclusive evidence than patterns that shi$. Pat- terns that show up with only one scheme may be important, but require special scrutiny (and an understanding of how the scheme works) to evaluate. AGGREGATED DATA: ENUMERATION VERSUS SAMPLES Quantitative data of the kinds depicted by the maps detailed in the previous section come from a diverse array of sources. In the U.S., one of the most important sources is the U.S. Bureau of the Census (discussed brie'y above). Here we focus in on one important distinction in data collected by the Census and by other organizations, a dis- tinction between complete enumeration (counting every entity) and sampling. Sixteen U.S. Marshals and 650 assistants conducted the !rst U.S. census in 1791. "ey counted some 3.9 million individuals, although as then-Secretary of State "omas Je#erson reported to President George Washington, the o&cial number understated the actual population by at least 2.5 percent (Roberts, 1994). By 1960, when the U.S. population had reached 179 million, it was no longer practical to have a census taker visit every household. "e Census Bureau then began to distribute questionnaires by mail. Of the 116 million households to which ques- tionnaires were sent in 2000, 72 percent responded by mail. A mostly-temporary sta# of over 800,000 was need- ed to visit the remaining households, and to produce the !nal count of 281,421,906. Using statistically reliable estimates produced from exhaustive follow-up surveys, the Bureau’s permanent sta# determined that the !nal count was accurate to within 1.6 percent of the actual number (although the count was less accurate for young and minority residences than it was for older and white residents). It was the largest and most accurate census to that time. (Interestingly, Congress insists that the original enumeration or “head count” be used as the o&cial population count, even though the estimate calculated from samples by Census Bureau statisticians is demon- strably more accurate.) As of this writing, some aspects of reporting from the decennial census of 2010 are still underway. Like 2000, the mail-in response rate was 72 percent. "e o&cial 2010 census count, by state, was de- livered to the U.S. Congress on December 21, 2010 (10 days prior to the mandated deadline). "e total count for the U.S. was 308,745,538, a 9.7% increase over 2000. In the !rst census, in 1791, census takers asked relatively few questions. "ey wanted to know the numbers of free persons, slaves, and free males over age 16, as well as the sex and race of each individual. (You can view replicas of historical census survey forms a t Ancestry.com) As the U.S. population has grown, and as its econo- my and government have expanded, the amount and variety of data collected has expanded accordingly. In the 2000 census, all 116 million U.S. households were asked six population questions (names, telephone numbers, sex, age and date of birth, Hispanic origin, and race), and one housing question (whether the residence is owned or rented). In addition, a statistical sample of one in six households received a “long form” that asked 46 more questions, including detailed housing characteristics, expenses, citizenship, military service, health problems, employment status, place of work, commuting, and income. From the sampled data the Census Bureau produced estimated data on all these variables for the entire population. In the parlance of the Census Bureau, data associated with questions asked of all households are called 100% data and data estimated from samples are called sample data. Both types of data are aggregated by various enu- meration areas, including census block, block group, tract, place, county, and state (see the illustration below). "rough 2000, the Census Bureau distributes the 100% data in a package called the “Summary File 1” (SF1) and the sample data as “Summary File 3” (SF3). In 2005, the Bureau launched a new project called American Com- Chapter 4: Mapping People with Census Data