Q. Hi. I have a question regarding the use of the em dash between two independent sentences (as in CMOS 6.91, last example). I understand that the dash can be used in place of a colon when introducing a list; however, when it is used to separate two independent sentences, as a semicolon would be used, it reads as a comma splice (as in your example): “The number of new cases has been declining—last week’s daily average was the lowest since January.” Wouldn’t a semicolon or period be better than an em dash in that example?
A. You’re right that a semicolon or a period might be better than a dash in that example. But we wanted our examples to show that a dash really can be used in place of just about any mark of punctuation. And don’t worry about creating a comma splice—only commas can do that.
It might help to show the same sentence but with parentheses:
The number of new cases has been declining (last week’s daily average was the lowest since January).
If you agree that those parentheses work, then consider that dashes and parentheses are usually interchangeable (though parentheses are not as abrupt as dashes, and they always come in pairs; see also CMOS 6.101).
But that doesn’t mean anything goes. Because dashes are so flexible, they tend to be overused. When in doubt, edit them out.