Census Data in Multicultural Populations?

In terms of sampling, especially for multicultural populations, census data seems to be pretty important in your process. It also seems like if you had a sample of 500 respondents, you would try to make sure at least 6% of them were Asian to represent the US. Can you talk more about the role of census data in your sampling for individual projects? Normally, we would use some census data as a guideline in sample design.