census itself. However, the ratio adjustments do not correct for other characteristics, such as income, on which the undercovered population might be expected to differ from the covered population in each adjustment cell.
There is substantial item nonresponse in the March income supplement. Household income is one such item for which nonresponse is substantial. Imputation techniques are used to provide values for people who fail to respond to the income supplement entirely, as well as for people who fail to answer one or more questions on the supplement.
Additional information can be found at the Census Bureau web site for the CPS (http://www.bls.census.gov/cps/ads/adsmain.htm).