This information was superseded in May 2013.
Explanation of Symbols:  Comment 

1. An '**' entry in the margin of error column indicates that either no sample observations or too few sample observations were available to compute a standard error and thus the margin of error. A statistical test is not appropriate.  
2. An '' entry in the estimate column indicates that either no sample observations or too few sample observations were available to compute an estimate, or a ratio of medians cannot be calculated because one or both of the median estimates falls in the lowest interval or upper interval of an openended distribution.  Also, note that this includes the scenario in which every value going into a median is the same value. The median is not released in this case; This is the same as in standard ACS products. 
3. An '' following a median estimate means the median falls in the lowest interval of an openended distribution.  
4. An '+' following a median estimate means the median falls in the upper interval of an openended distribution.  
5. An '***' entry in the margin of error column indicates that the median falls in the lowest interval or upper interval of an openended distribution. A statistical test is not appropriate.  
6. An '*****' entry in the margin of error column indicates that the estimate is controlled. A statistical test for sampling variability is not appropriate.  
7. An 'N' entry in the estimate and margin of error columns indicates that data for this geographic area cannot be displayed because the number of sample cases is too small.  Will not get this in this tab. 
8. An '(X)' means that the estimate is not applicable or not available.  Will not get this in this tab. 
9. An '^' means the estimate is invalid (not logically possible in the dataset).  Special symbol used in CTPP special tabulation. 
Data source citation for tables from the CTPP should be:  
U.S. Census Bureau, American Community Survey 20062010 Fiveyear estimates, Special Tabulation: Census Transportation Planning Products.  
Information on the ACS:  
http://www.census.gov/programssurveys/acs/about.html  
Design and Methodology Report (April 2009):  
http://www.census.gov/programssurveys/acs/methodology.html  
Errata notes on the ACS:  
http://www.census.gov/programssurveys/acs/technicaldocumentation/errata.html  
Median interpolation method description:  
Median travel time (minutes to work) is calculated using linear interpolation. Median income figures are calculated using Pareto interpolation. Each median is calculated using a standard distribution: 15 bins are used for median travel time (minutes to work) and 39 bins are used for median household income (dollars).  
Excerpts of ACS Special Tabulation Rules (April 23, 2008):  
2. All cells in any American Community Survey special tabulation must be rounded. The rounding schematic for all tables is: 0 remains 0 17 rounds to 4 8 or greater rounds to nearest multiple of 5 (i.e., 864 rounds to 865, 982 rounds to 980) Any number that already ends in 5 or 0 stays as is. Any totals or subtotals needed should be constructed before rounding. This assures that universes remain the same from table to table, and it is recognized that cells in a table will no longer be additive after rounding. 

3. Medians or other quantiles may be calculated as A. an interpolation from a frequency distribution of unrounded data (these are not subject to additional rounding), 

5. Percents, rates, etc., should be calculated after rounding, but the DRB has granted exceptions to this rule when the numerator and/or denominator of the percent or rate is not shown.  
6. Means and aggregates must be based on at least 3 values.  
Excerpt of CTPP 20062008 Special Tabulation Rules (April 22, 2009):  
For all flow tables, there must be at least 3 unweighted cases per pair of geographic areas with the exception of Total Workers univariate and Means of Transportation univariate (no required threshold). 