Simple stupid example. A lot more than 1/5 of the people would choose A for the following question:
How many people have you killed?
a) 0
b) 1-9
c) 10-99
d) 100-999
e) 1000 or more
How many people have you killed?
a) 0
b) 1-9
c) 10-99
d) 100-999
e) 1000 or more
Do you have a Y chromosome? a) Yes b) No
Do you have two X chromosomes: a) Yes b) No
Each are 50-50 questions, but the answer to A is correlated to the answer to B.
It's hard to find questions whose answers are independent variables.
A) how many people don't understand the questions
B) how many people understand the questions but don't choose to answer "correctly" e.g. transgender. E.g. smart arse Yes, Yes answers.
C) correlation with a third variable e.g. sampling bias (HN readers that answer internet surveys).