Missing data for a particular question or item is called item nonresponse. It occurs when a respondent fails to provide an answer to a required item. The ACS also considers invalid answers as item nonresponse. The Census Bureau uses imputation methods that either use rules to determine acceptable answers or use answers from similar housing units or people who provided the item information. One type of imputation, allocation, involves using statistical procedures, such as within-household or nearest neighbor matrices populated by donors, to impute for missing values.

From 1999-2007, in the American Community Survey, people reported to be enrolled in "public school, public college" or "private school, private college" were classified by grade or level according to responses to Question 10b, "What grade or level was this person attending?" Seven levels were identified: "nursery school, preschool;" "kindergarten;" "elementary grade 1 to grade 4" or "grade 5 to grade 8;" "high school grade 9 to grade 12;" "college undergraduate years (freshman to senior);" and "graduate or professional school" (for example: medical, dental, or law school).

In 2008, the school enrollment questions had several changes. Home school was explicitly included in the private school, private college category. For question 10b the categories changed to the following "Nursery school, preschool," "Kindergarten," "Grade 1 through grade 12," "College undergraduate years (freshman to senior)," "Graduate or professional school beyond a bachelor's degree" (for example: MA or PhD program, or medical or law school). The survey question allowed a write-in for the grades enrolled from 1-12.