The U.S. Census Bureau has spent years figuring out how to phrase the six questions they'll ask everyone in 2010.
Question No. 3 asks gender, with the admonition to "Mark ONE box" -- male or female. Whether the Census Bureau included that instruction or left it out in the 2005 field test, the results were the same. Either way, 0.05% of those asked -- that would mean 150,000 in a population of 300 million -- still checked both. But the instruction made the question longer and more visible, and fewer people skipped it.
Question No. 4 asks age -- and for a computer double-check, date of birth -- because so many people seem to get it wrong. Adding instructions to "report babies as age 0" when they're less than a year old, offends some people, census research suggests. But in the 2005 trial it improved the response rate among people who otherwise couldn't decide how to answer for a six-month old. [Link (reg. req.)]