Census has ACS data starting 2005, but in IPUMS ACS goes further back?

How are the ACS data going further back than 2005 constucted in IPUMS? I see that in Census the availabe ACS data goes back until 2005 ? I wanted to use ACS 2000 and compare it to ACS 2016 but not sure if they are comparable?

It sounds like you may be looking at what the Census currently has available through summary files on their website. You can obtain ACS data through the Census Bureau back to 2000 through their FTP site. As such, there is nothing inherently different about the way ACS data from 2000 - 2004 through IPUMS USA is constructed. Just as in the samples from 2005 - 2016, there are comparability notes with each variable that will highlight differences across time. One set of variables that is particularly subject to comparability issues between 2000 and 2016 is the set of geographic variables. For example, PUMA, a variable identifies the Public Use Microdata Area (PUMA) where the housing unit was located, is not comparable across years because the Census Bureau redraws PUMA boundaries every 10 years based on population information gathered from the most recent decennial census.

I would recommend that in addition to individual sample notes here, that you read the description, comparability, and universe tabs available for each of your variables of interest. This should inform you of any comparability issues. And of course, if you have additional questions, please reach out to us.