The Physics Factbook is supposed to be an encyclopedia of student essays. What better way to start than by referring to a real encyclopedia. Get out of the children's section and get into a real library. Sometimes old resources get to the point quicker or explain things more simply. Sometimes they are completely wrong.
Students automatically assume that anything old is outdated.
Since there is no definitive answer, maybe there are no definitive resources. The students are then given two weeks to a month to research their assigned measurements and synthesize their findings about it. Not every student will produce an accurate, articulate synthesis of their research on the first try. Many times they will say something obviously wrong as they think there must be an answer. Sometimes, they have no idea what they are doing.