Multiracial Population Respondents can be counted as multiracial one of three ways: First (self), if they select two or more races for themselves. Second (parents), if they do not select two or more races for themse Ives but report that at least one or their biological parents was not the same race as them, or select two or more races for at least one of their parents. Third grandparents), if they do not fit the definition of multiracial
Part of the reason behind the absence of data on this group is the size of the multiracial population – it’s still relatively small. As you note, we estimate that only about 6.9% of the adult population has a mixed-race background. And by the Census Bureau’s definition the share is just 2.1%. That means even large, nationally representative surveys typically include too few mixed-race adults to analyze, much less contain sufficiently sized samples of individual multiracial groups such as white and black or white and Asian biracial adults.
Estimating the Size of the  | Multiracial Population  | Multiracial  | 2.6%  | background based  | on grandparents  | 6.9%  | Multiracial  | 2.9  | background based  | on parents  | Chooses two or more  | 1.4  | races to describe self  | Respondents can be counted as multiracial one of three ways:  | First (self), if they select two or more races for themselves.  | Second (parents), if they do not select two or more races for  | themse Ives but report that at least one or their biological parents  | was not the same race as them, or select two or more races for at  | least one of their parents.  | Third grandparents), if they do not fit the definition of multiracial  | based on their own or their parents' racia background, but indicate  | that at least one of their grandparents was not the same race as  | themsel Ives or their parents or select two or more races for at least  | one of their grandparents.  | Source: Pew Research Center survey, Feb. 6-April 6, 2015  | (n=21 224 sampled adults)  | PEW RESEARCH CENTER  | R1-4 