In order for the results of a survey to be valid, the sample used has to be both random and representative.
Typically this is obtained via a stratified random sample.
A stratified random sample is a subset of the population that is drawn in such a way that every member of the population and important sub-categories of the population have an equal chance of being selected for the survey, yielding a sample that is demographically similar to the population.
A convenience sample is not considered rigorous or accurate—it results in a non-representative sample.
Generalizability means that the results from the sample can be assumed to apply to the general population with confidence.
Nye conducted formal interviews of 780 young people in Washington State, though his sample was criticized for not including individuals from urban backgrounds and for only selecting individuals who were likely to describe their families unfavorably.