The only thing it would mean to me is a proportion. "On the average, three and a half of a billion people inherit the ABCD gene from both parents." (Obviously, there is no such thing as half a person, but in statistics that doesn't matter. People often say things like "the average family has 2.5 children and 0.3 dogs.") That might not be the best way to say this - I'd probably say "about one person in 290 million" - but that's what it would mean to me.Hmm... interesting. What do you think "three and a half of a billion" means?