The authors included Part 4 to provide proof that people consider individuals with disabilities inferior to them and thus the majority of people thinks they shouldn't have the same rights as them. The Census proves that because it is a collection of surveys where people clearly stated their opinions about people with disabilities.
It is related to the claim because it provides evidence that people generally discriminate against those with disabilities and aren't even trying that hard to help them, when they should include them in the society and live together and equally.
