You are interested in the emergency of substances which are not yet regulated or illegal, but cause significant intoxication or an altered state of mind. You are one of the first researchers to examine salvia divinorum use by teenagers. In this ______ study, your team of researchers observe the number of teenagers who visit six "head shops" recently featured in High Times magazine as having the widest selection of salvia products in LA county. You plan to count the number of teenagers visiting such shops and record the amount of time each teen is inside to assess the scope of salvia use in a small geographical area believed to have high rates of use. In order to conduct this _______, you train a team of research assistant to identity potential subjects into the ________ categories of pre-teen (aged 10-12), early teen (13-15), middle teen (16-17), and late teen (18-19). Next, you send a team of two research assistants, each of whom separately record subjects' age, to each of the head shops to achieve _______ in assessing age. To simplify your IRB compliance, you decide to pursue your study further using only a sample of late teen (18-19-year-old) subjects, because younger teens are ________. After two weeks of observation, you identify two stores with the highest numbers of customers who appear to be in the late teenage years. You then send researchers to strike up casual conversations with the potential subjects in these two stores. After an initial conversation about salvia, the research assistants tell the potential subjects about the study and obtain ______.