There are many differences in the two surveys. Most important to consider are:
In Census 2000, about 7.8 million people lived in Group Quarters, and 1 million were workers. 2005 ACS does NOT include population who live in Group Quarters.
1990 and 2000 decennial Census worker counts included in these profiles include Group Quarters.
Because the sample size of the American Community Survey is much smaller than the decennial census "long form," it is more important to understand the potential errors in the tabulated results.
Calculation of Standard errors for some of the data elements on these profiles are based on the Census Bureau method detailed at http://www.census.gov/acs/www/Downloads/data_documentation/Statistical_Testing/ACS_2005_Statistical_Testing.doc
These have been expressed on the profile sheets as "Margins of Error" either as a +/- of a count, or of a percentage, using a 90 percent confidence interval.
Because the Margin of Error for "Total Employed" needed to be calculated from a table with six variables, the resultant values were over-estimates. We decided not to show them on the profile sheets.
The 2005 ACS data are collected over all twelve (12) months of the year.
The decennial census data for 1990 and 2000 are collected for "April 1" of the decennial year.
Areas with large seasonal population shifts, e.g. "snow bird" populations, migrant workers, college/university enrollments are likely to see the greatest differences between decennial and ACS results.
Both the decennial Census and ACS do not include people who take transit occasionally. The question on mode to work asks "how did this person usually get to work last week?" For an examination of usual mode to work and actual mode to work , please see http://www.trb.org/conferences/censusdata/Resource-Journey-to-Work.pdf, Table 4, Page 10.
MSA data in these profiles are based on the November 2004 definition of Core Based Statistical Areas, and Consolidated Statistical Areas. 1990 and 2000 Census data at the county level were used to prepare MSA level data based on the November 2004 definition. A bridge between the old and new definitions are posted at http://www.census.gov/population/www/metroareas/files/CBSA03_MSA99.xls
Maps of MSAs are posted at http://www.census.gov/geo/www/maps/stcbsa_pg/stBased_200411_nov.htm