Pollster: When opinion researchers need a population sample that reflects the demographic characteristics of the national population, they choose their sample on the basis of national census data  Not everyone participates in the national census, despite its being mandatory  If, however, census participation became voluntary, as some have proposed, the participation rate would be much lower
So if census participation became voluntary, polls designed to discover the opinions of the national population would have less accurate results
The group of people who would participate in a voluntary national census would differ in its demographic characteristics from the group of people who would participate in a mandatory national census.