http://worldversus.com/Cocacola-vs-Pepsi
Coca-Cola had 14,490 people vote- %67.28
Pepsi had 7046 people vote- %32.72
This vote is voluntary response because they have a vote and ask people to participate. The people have a choice whether to vote or not to vote so it is a voluntary response. The website has many other votes that people can chose to participate in.