title: Differing results for efficacy--sampling bias or purposive sampling?
authors: Francesco
authors: F
authors: Mercorio
mesh_terms: D003271:Contraceptive Agents, Female / Q000008:administration & dosage*; D005260:Female; D006801:Humans; D007434:Intrauterine Devices; D007889:Leiomyoma / Q000150:complications*; D016912:Levonorgestrel / Q000008:administration & dosage*; D015983:Selection Bias; D016896:Treatment Outcome; D014592:Uterine Hemorrhage / Q000188:drug therapy* / Q000453:epidemiology / Q000209:etiology*; D014594:Uterine Neoplasms / Q000150:complications*
chemical_list: D003271:Contraceptive Agents, Female; D016912:Levonorgestrel