Voluntary Response Sampling:

• Individuals choose to be involved. These samples are very susceptible to being biased because different people are motivated to respond or not. Often called “public opinion polls,” these are not considered valid or scientific.
• Bias: Sample design systematically favors a particular outcome.

Ann Landers summarizing responses of readers

70% of (10,000) parents wrote in to say that having kids was not worth it—if they had to do it over again, they wouldn’t.

Bias: Most letters to newspapers are written by disgruntled people. A random sample showed that 91% of parents WOULD have kids again.