ACS Data resources for 2001-2004

Dear who may know the answers,
I think the first year that ACS became available was 2005. I also checked that the sample size for the year before 2005 and after are quiet different. But the IPUMS shows that it has the ACS data for the year 2000-2004 from Census Bureau. Then would there contain any difference between these two?

You can think of the 2000-2004 ACS data as the beta ACS; data collection for the ACS was ramping up in these years. Because of this, the sample sizes are much smaller, the data do not include geographic identifiers below the state, and do not include persons in group quarters. A bit more information about these samples is available on the IPUMS USA sample description page and the IPUMS USA description of the ACS.