title: Differences in volunteer and randomly acquired samples.
authors: M E
authors: ME
authors: Wewers
authors: K
authors: K
authors: Ahijevych
mesh_terms: D005260:Female; D006801:Humans; D008297:Male; D008402:Mass Media; D015399:Nursing Research / Q000379:methods / Q000592:standards*; D011897:Random Allocation; D015983:Selection Bias; D000074606:Smoking Prevention