Notes on Data Sources
In the past, demographic data was largely provided by the US Census Bureau which up until recently, only gathered and published information once every ten years. However, the US Census Bureau now has a number of initiatives that gather and publish data on an ongoing basis. There are also other organizations that gather, calculate, or project demographic and other data. The data presented within this catalog have been gathered form the following sources:
County and Regional Data
The Chester County Planning Commission extracts data from Chester County tax assessment records relating to the physical features and prices of housing and real estate. This information is then sorted and used to generate housing and real estate data relating to municipalities and the County as a whole.
The Delaware Valley Regional Planning Commission (DVRPC) extracts data from the US Census Bureau and other sources to generate future projections of population and other demographic and economic features. The DVRPC also generates land use data for Counties and municipalities in and around Philadelphia based on interpretations of aerial photography. The DVRPC typically updates it data sets at least every five years.
Federal US Census Data Sources
The US Census Bureau's Decennial Census is an actual field count, called an "enumeration," conducted on every year ending with a "0." It enumerates all persons within the United States as of April 1 of the census year. The Decennial Census is not an estimate, but rather a capture of data that relates to but one day, every ten years. From 1940 through 1990, the Decennial Census gathered information on population and housing using two forms: the short-form which was filled out by each household, and the long-form which included more detailed questions and was sent to a much smaller sample. The Decennial Census long-form was discontinued in 2010, when the American Community Survey was initiated.
The US Census Bureau's 5-year American Community Survey (ACS) is an estimate based on the results of survey questionnaires sent out to households over a five year period. These results are then used in statistical calculations that generate estimates, not actual enumerated counts. For example, the 2015 ACS gathered information from a sample of households each year, and then used that raw data to generate estimates that related to the five year period from the first day of 2001 to the last day of 2015. Thus, the 2015 ACS data cannot be compared with data gathered only in 2015, since the 5-year ACS data covered all five years (from 2011 through 2015). However, the 5-year ACS data of 2015 would be most reflective of (or a closet match to) data gathered only in 2013, because 2013 is the middle year of 5-year ACS period (2011, 2012, 2013, 2014, and 2015). The 5-Year ACS estimates are generated for the nation, and for each state, county, municipality, and census block. The results for any year are usually published the following year, such that the 5-year ACS of 2015 was published in 2016.
The US Census Bureau's 1-year American Community Survey (ACS) is an estimate based on the results of survey questionnaires sent out to households over a 12-month period within one calendar year. These results are then used in statistical calculations that generate estimates, not actual enumerated counts. For example, the 2012 ACS gathered information from a sample of householders each month, and then used that raw data to generate estimates that related to the 12-month period from January 1 to December 31, 2012. Thus, the 1-year ACS data for 2012 can be reasonably compared with other data gathered in 2012. The 1-Year ACS estimates are generated for the nation, and for each state, but only for jurisdictions with a population of at least 65,000 people. As a result, these data are not available for some counties and most municipalities. The estimates for any year are usually published the following year, such that the 1-year ACS of 2012 was published in 2013.
The US Census Bureau's Population Estimates Program (PEP) Population Projections are estimates generated for each of the nine years following the Decennial Census. Each of these projections is generated using the Decennial Census as the base year. Thus, the 2011 projection was published one year after the 2010 Decennial Census. One year later, the 2011 and 2012 projections were published. Projections that are based on the 2010 Decennial Census will continue to be published until 2019. After that, the 2020 Decennial Census will become the base year from which the 2021 through 2029 projections are generated. US Census Bureau's Population Projections for a given year will not always match projections made by other government or private organizations, and may even be different form 1-year ACS data.