One form of error that can affect census adjustments is correlation bias, reflecting people who are doubly missing-- from the census and from the adjusted counts as well. This paper presents a method for estimating the total national number of doubly-missing people and their distribution by race and sex. Application to the 1990 U.S. census adjustment leads to an estimate of 3 million doubly-missing people. Correlation bias is likely to be a serious problem for census adjustment in 2000. The methods of this paper are well suited for measuring its magnitude.