Recent trends in income segregation in metropolitan regions show that, after a decline in the 1990s, there was an increase in 2000–2010 that reinforced concerns about the overall growth in U.S. income inequality since the 1970s. Yet the evidence may be systematically biased to exacerbate the upward trend because the effective sample for the American Community Survey (ACS) is much smaller than it was for the 2000 census to which it is being compared. Apparent changes in disparities across census tracts may result partly from a higher level of sampling variation and bias due to the smaller sample. This study uses 100% microdata from the 1940 census to simulate the impact of different sampling rates and applies those approaches to publicly available data for 2000 and 2007–11. The reduction in sample sizes associated with the ACS appears to exaggerate the evidence for increasing income segregation for all measures tested here.