Is there any issue I should be aware of when working with ACS data from 2001 to 2004? It is my understanding that the ACS was not fully implemented until 2005, I am wondering if the data pre-2005 is reliable and comparable with post-2005 data.
You are correct in that the ACS was not fully implemented until 2005. Prior to that, only a limited number of counties were sampled. While the pre-2005 data are weighted to maximize representativeness, these samples are not fully comparable with the 2005-onward data. Please see Chapter 2 of Technical Paper 67 for more details.