ACS Overview and Comparison to 2000 Census Data.

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ACS Overview and Comparison to 2000 Census Data.
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What do you need to consider when comparing data
from the 2005 ACS with Census 2000 data?
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Two factors to consider

• Interview and residence rules what is the
  target population?
• Time period and reference period what period of
  time are the data describing?

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ACS Target Population

• Housing unit population of the United States and
  Puerto Rico
• Current residents

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ACS Target Population

• Whenever possible, compare household population
  to household population
• When you dont have estimates for the household
  population for 2000, assess the likely impact of
  the GQ population

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Residence Rules

• Must work for all types of areas
• Must take into account changes in populations
  over the course of the year

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Goal of residence rules

• Resident of the sample address if a person
• Lives there year round
• Lives there more than 2 months but not year round
• Is living there now with no other place to live
• Is away now for 2 months or less

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Goal of residence rules

• Not a resident of the sample address if a person
• Lives there 2 months or less with another
  residence
• Is away now for more than 2 months

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Would the residence rules include
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Would the residence rules include
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• ACS data will describe a population similar to a
  census usual resident population if the area
• has a relatively stable population throughout the
  year
• includes seasonal populations who only stay for
  short periods of time (2 months or less)
• includes trivial numbers of long-term seasonal
  populations

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• ACS data will describe a population that differs
  from a census usual resident population
• if the area includes a large proportion of
  long-term seasonal populations
• if the population of the area changed
  dramatically in the course of the year

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What to expect in 2005
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ACS produces period estimates

• They describe the characteristics of an area over
  a specific time period
• Contrast with point-in-time estimates that
  describe the characteristics of an area on a
  specific date

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Weighted Census 2000 Sample 2000 ACS Housing
Units by Response Week
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Census 2000 Data Collection Design
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What do the ACS estimates represent?

• ACS data collection is a sequential process of 3
  operations on each of 12 monthly samples over 3
  month periods
• All 3 operations take place every month on
  different sample panels, a continuous series of
  interviews
• Data collected in a calendar year are grouped by
  interview month a weighting adjustment is
  applied to smooth overall monthly differences
• Summed estimates are considered the average
  characteristics of all areas for the calendar year

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How does data collection compare?

• Differences
• ACS nonresponse FU uses CATI and CAPI
  instruments past censuses have used only paper
  questionnaires
• ACS data collected only from household members
  census data often collected from neighbors
• ACS interviews conducted by experienced and
  well-supervised staff census enumerations
  conducted by inexperienced temporary workforce

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ACS Data Collection Design
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How do the reference periods used in the
questions on the ACS impact the interpretation of
results?
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Reference Periods

• ACS uses the interview date as the single
  reference point, or as the end of a reference
  period, for all data collection
• The Census 2000 sample did too, except for
  income, migration, and agricultural sales

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Questions with no specific reference period

• Most ACS questions do not stipulate a period of
  time that should be referenced
• Interpretation is yearly average since the data
  are collected each month and averaged across
  months

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Questions with a specific reference period
relative to interview date

• Other questions specify a period of time relative
  to the date of interview
• Interpretation is still a yearly average but
  covering a slightly different period of time than
  the calendar year

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Illustration of an ACS Release schedule
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So, what does all this mean about making
comparisons back to Census 2000?
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Comparisons with Census 2000

• Four reports by Bureau staff compared single-year
  data from the ACS with Census 2000 at national
  level
• Two staff reports and four reports by outside
  researchers compared three-year averages from the
  1999-2001 ACS test sites with Census 2000 at
  county and tract levels

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Comparisons with Census 2000

• Please refer to this website for results of these
  comparisons and analysis of the differences
  between the ACS and Census 2000
• www.census.gov/acs/www/advMeth/Reports

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Its OK to compare when

• The target populations are the same or similar
• if tables include the same universe
• if areas have small GQ populations or
  insignificant seasonal populations

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Its OK to compare when

• The questions havent changed significantly or
  the reference period is essentially the same