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Sample Size -- Definitions
1. How large is the sample?
The full implementation ACS which began in 2005 samples approximately 3 million housing unit addresses annually stateside. The PRCS samples approximately 36,000 housing units addresses each year in Puerto Rico.
The full implementation ACS and PRCS Group Quarters which began in 2006, samples approximately 200,000 people living in group quarters annually.
The ACS sampled between 740,000 and 900,000 addresses annually in 2000 through 2004.
2. What is sampling error?
ACS estimates are based on data from a sample rather than from all housing units or people living in group quarters in the population. For this reason, ACS estimates have a degree of uncertainty associated with them, called sampling error. In general, the larger the sample, the smaller the level of sampling error.
3. Why is it important to measure sampling error?
The estimates produced by the ACS are not exact because they are based on a sample. The sampling error measures the degree of uncertainty associated with the estimates. If the degree of uncertainty is too large, users should be cautious in how the estimates are used.
4. How does the ACS measure sampling error?
The ACS calculates standard errors for each estimate produced and publishes the 90 percent confidence level margin of error (the Census Bureau standard). You can be 90 percent confident that the interval within the margin of error from the estimate includes the true value. See Accuracy of the Data and Understanding the Change Profiles under the section "Using the Data" for more details on how margin of error and confidence intervals are calculated and interpreted.
5. What other numbers provide important information on the reliability of ACS estimates?
Housing Units Initial Addresses Selected - The number of addresses in each state and for the nation that were selected for the ACS sample for a particular year. Each year's sample is systematically divided into 12 monthly samples for ACS interviewing. This initial number includes addresses later determined to be commercial or nonexistent, as well as housing units that are not interviewed due to subsampling for personal visit follow-up, refusals or other reasons.
Housing Units Final Interviews - The final number of interviews across all three modes of data collection for the ACS in a given year for the nation and by state. This number includes occupied and vacant housing units that were interviewed by mail, telephone, or personal visit methods between January 1 - December 31. It excludes addresses determined to be nonexistent or commercial, and addresses not selected in the subsample for personal visit follow-up, and addresses that are not interviewed due to refusals or other reasons.
Group Quarters People Initial Sample Selected - The number of people living in GQs that could be contacted for ACS interviewing in a given year for the nation and by state. Each year's sample is systematically divided into 12 monthly samples for ACS interviewing. This initial number includes people thought to be in group quarters that were later determined to be out of scope or nonexistent, as well as people not interviewed due to the group quarter refusing entry, the person refusing to respond, or other reasons.
Group Quarters People Final Interviews - The final number of person interviews for the ACS for those living in group quarters in a given year for the nation and by state.
6. Are these numbers used to calculate the survey response rate?
These numbers by themselves are not used to measure the response to the request to participate in the ACS. See Survey Response Rates - Definitions for more information.
7. How does the ACS calculate these four measures of sample size?
| Housing Units Initial Addresses Selected (state x, year y) = | | the sum of the 12 monthly address samples selected from the Master Address File for state x in year y.
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| Housing Units Final Interviews (state x, year y) = | | the sum of all interviews successfully conducted by mail, telephone, or personal visit between January 1 and December 31 of year y, for state x.
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| Group Quarters People Initial Sample Selected (state x, year y) = | | the sum of people living in group quarters that we did contact and those we expected to contact over the 12 months for state x in year y.
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| Group Quarter People Final Interviews (state x, year y) = | | the sum of all interviews successfully conducted by personal visit between January 1 and December 31 of year y, in state x.
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