They don’t. For example, look at this mess:
nbcnews.com/id/23574940/ns/health-childrens_health/t/teen-girls-has-sexually-transmitted-disease/
The study was done at inner city public health centers, so when they say “1 in 4 teen girls have a STD,” they really mean “1 in 4 teen girls who volunteerly entered a public health center, designed for the poor in the inner city, have an STD,” which really is not sensational. And that’s just one problem with the article!
Nope
Yes! The study basically allows the respondent to define the term “Christian.” Those who are pro-choice can claim to be Christians, for example (like the President and Vice President). A study loses much usefulness in not defining terms objectively, but on the other hand, the point of this study doesn’t seem to be to find who truely is a Christian, but to find who identified as a Christian. (This lack of an objective definition for who a Christian is stems from Protestantism, as anyone who claim Divine inspiration can call himself a Christian, and who a Christian is ultimately defined by any person, which makes Christianity undefined. But I regress from the topic

).
Yes, it takes a long time to write a detailed and satisfactory post on the Internet
Christi pax,
Lucretius