InStyle magazine wants to examine the pull power of two types of copy in increasing the level of newsstand sales for a given month. The issue will feature British royal Kate Middleton but InStyle wants to know whether newsstand sales will be higher for a cover that mentions Middleton's daily life or one that promises to share Middleton's style secrets. The issues are identical with the exception of the copy underneath the cover graphic of Middleton. Which type of study is this an example of?