Arnold wants to know if portrayals of the elderly in television commercials have become more negative over time. to this end, he obtains a collection of television commercials from the 1950s to present day. he trains his research assistants to code the presence/absence of elderly people in the commercials, as well as the way in which they are portrayed (e. g., competent, confused, active, sickly, cranky, and conservative). what sort of study is this?