Q. I’ve always followed this advice in Chicago: “If, as occasionally happens, the Collegiate disagrees with the Third International, the Collegiate (or its online counterpart) should be followed, since it represents newer lexical research.” We subscribe to the online Unabridged (which also includes the Collegiate), and lately this advice no longer seems to apply consistently. Merriam-Webster seems to be updating entries in the Unabridged and leaving the Collegiate with the older version. For example, the Unabridged has life-span while the Collegiate has life span. Typically, the hyphenated version would be the more up to date.
A. It’s true—the lexicographers at M-W can’t be everywhere at once, which leaves discrepancies between their various versions. But the kinds of changes you’re talking about are minor. It’s not as though life span is now so grossly incorrect that using it would invite viral shaming on Twitter. We hyphenate a compound to make it easier to read or to prevent misreading. If you use common sense and keep a style sheet, you needn’t worry about whether you’re up to the minute with M-W.