The Census Bureau publishes data for 1-year, 3-year, and 5-year estimates, with population thresholds set for the 1-year and 3-year estimates to produce reliable data. Here is a brief comparison of the three types of estimates:

1-Year Estimates	3-Year Estimates	5-Year Estimates
Published for selected geographic areas with populations of 65,000 orgreater	Published for selected geographic areas with populations of 20,000 or greater	Published for all geographic areas including those with a population under 20,000.
Represent the average characteristics over a calendar year	Represent the average characteristics over the 3-year period of time	Represent the average characteristics over the 5-year period of time
Have fewer published geographic areas than the 3-year and 5-year estimates	Have more published geographic areas than the 1-year estimates but fewer than the 5-year estimates	Have more published areas than the 1-year and 3-year estimates

For more information on the ACS estimates, users are encouraged to visit the ACS website http://www.census.gov/acs/www/guidance_for_data_users/estimates/.

For data quality and geographic reasons, some tables are published with specific geography restrictions. For example, the able B05001-PR, Citizenship Status in Puerto Rico, is produced for Puerto Rico only, and the table B08501, Means Of Transportation To Work By Age For Workplace Geography, is only produced for the work place geographies. Appendix E has a complete list of tables and their geography restrictions, and Appendix F has a complete list of ACS 1-year published summary levels and components. Other examples include the quality measures tables (the B98 series) and detailed occupation tables.

Some data values represent unique situations where either the information to be conveyed is an explanation for the absence of data, represented by a symbol in the data display, such as "(X)", or the information to be conveyed is an open-ended distribution, such as 115 or greater, represented by 115+.

The following special data values can appear in the ACS Summary File table as an explanation for the absence of data:

Missing Value = ""

A missing string indicates that the estimate is unavailable. (This appears in the estimates and margins or error files as two commas adjacent to each other without anything between them, or if the last cell in a data file is filtered then you get a comma followed immediately by a carriage return or EOF.) A missing value indicates when an estimate is missing because of filtering for geographic restrictions, coefficients of variations (CV), or was removed due to the Disclosure Review Board's (DRB) requirements. For more detail on filtering, please see Appendix C.5.

Dot = "."

A dot indicates when the estimate has no sample observations or too few sample observations. In the margin of error files, this value could also indicate that the margin of error is unavailable for a median estimate that has been replaced with a jam value.

Zero = "0"

A "0" entry in the margin of error column indicates that the estimate is controlled. A statistical test for sampling variability is not appropriate. This is similar to the "*****" symbol used in American FactFinder.

Negative 1 = "-1"

This indicates that an estimate does not contain a Margin of Error. Tables B00001, B00002, and tables starting with B98 and B99 do not have margin of error (MOE) associated with them. The MOE calculations are set to -1 for these tables.

B00001, B00002, B98001, and B98002 are sample counts, not estimates, and do not have margin of error (MOE) associated with them. Tables in series B99* imputation tables and B98* quality measure tables do not provide margin of error calculations. The margin of error calculations are set to -1 for these tables.
There are a few special rules on how certain margin of error are determined for ACS estimates. The accuracy of the estimate (decimal place) within the detailed tables determines how many digits the margin of error is rounded.

Filtering rules, based on statistical reliability of the survey estimates, are used because certain geographic areas contain detailed tables include estimates whose level of reliability is unacceptable. The data release rules for the American Community Survey data tables include the following.
Every base table consists of a series of estimates. If more than half the estimates are not statistically different from 0 (at a 90 percent confidence level), then the table fails. Each estimate is subject to sampling variability that is summarized by its standard error. Dividing the standard error by the estimate yields the coefficient of variation (CV) for each of the estimates. (If the estimate is 0, a CV of 100 percent is assigned.) To implement this requirement for each table at a given geographic area, CVs are calculated for each of the table's estimates, and the median CV value is determined. If the median CV value for the table is less than or equal to 61 percent, the table passes for that geographic area; if it is greater than 61 percent, the table fails. Tables that are too sparse will fail this test. In that case, the table will not be published for that geographic area. Whenever a table fails, a simpler table that collapses some of the detailed lines together can be substituted for the original, more detailed table. The rules are then applied to the simpler table. If it passes, the simpler table is released. If it fails, none of the estimates for that particular table is released for this geographic area. These rules are applied to single-year period estimates and multi-year period estimates based on three years of sample data.

The estimates in the summary files are stored using standard notation instead of in scientific notation. The estimates are stored as whole numbers. The largest estimate in a Summary File contains 14 digits.

There are eight tables with more than the maximum 245 cells that cannot fit into a single sequence, so each of these tables is broken into multiple sequence files. The table below shows the tables that contain multiple sequences:

Data users often question why certain American Community Survey (ACS) estimates are not available. Missing estimates can be caused by data suppression through various methods or restrictions that are applied to ACS data to limit the disclosure of information about individual respondents and to reduce estimates with unacceptable statistical reliability. The factors contributing to the data suppression include:

• Population thresholds (more information in section CI)
• Geographic restrictions for tables (more information in section C2)
• Data quality filtering (more information in section C5)
• Collapsed tables: ACS produces and releases two types of Detailed Tables, base tables and collapsed tables. The collapsed tables were introduced in an attempt to release more data to the public because many of the corresponding base tables were failing data quality filtering. Users may see the collapsed tables without counterpart detailed tables.
• DRB rules: Disclosure Review Board (DRB) establishes additional rules that specify what ACS data are released to ensure that the confidentiality of the individual's data are protected.
• Table cells and table topic restrictions: Table with more than 100 independent cells or tables with sensitive topics that could lead to disclosure risks are not allowed at the block group level.
• Table suppression for specific geographies: During the data review period, subject matter analysts may identify errors in microdata which occurred during data collection or early data processing and impacted tables and associated geographic areas. To avoid releasing unreliable estimates, one method used is to suppress tables for affected geographic areas at the last stage of the dissemination from being released to the public. Table suppression is documented in errata notes on the ACS Website on:

Additional information concerning data suppression is available on the ACS website on: http://www.census.gov/acs/www/