One of the key methods used to do political polling is random-digit dialing, in which every phone number in an area code has an equal chance of being selected to take part in a survey. however, researchers have noticed that young people are more likely to only use a cell phone, and people with cell phones are less likely to answer a call from an unknown number. as a result, polling organizations often count responses from young people as being worth "more" than those from older people so that age in the sample is more accurately representative of the larger population. what is this technique called?