Voluntary response is not random and therefore is not included.
Work Step by Step
The voluntary response sample is not included in the study because the sample obtained was not random and may have introduced self-selection bias. Suppose you used sound statistical techniques to analyze voluntary response samples, the results still would not be valid. For this reason the voluntary response sample is not included in the study. Think of American Idol, the voters are voting for their favorite contestants based on their voluntary response sample involvement.