Your latest issue of cosmopolitan includes a survey of readers on jealousy. you are asked to mail in your responses, and 19,000 responses are received. the results appear in the next issue. the results cannot validly be generalized to a population beyond the readership of cosmo because

The results cannot validly be generalized to a population beyond the readership to Cosmo because "The sample is self-selected".

A sample is self-selected when the consideration or avoidance of inspecting units is dictated by whether the units themselves concur or decrease to take an interest in the example, either unequivocally or verifiably.

Self-selection sampling is a non-likelihood procedure, that depends on the judgment of the analyst. This is a valuable apparatus for specialists, who need individuals or associations (units), to partake (or volunteer) as a major aspect of an investigation voluntarily.