1. “Thanks to technology, we can have independence, relative independence, from the harsh qualities of the real world on a day-to-day basis.”
This sentence’s punctuation — a series of three commas — implies a flat progression of ideas without modulation. But the writer, after the fact, modifies the absolute word independence with the qualifying term relative, and should signal this slight case of backpedaling by marking the phrase “relative independence” as an interjection: “Thanks to technology, we can have independence — relative independence — from the harsh qualities of the real world on a day-to-day basis.”
2. “You, yes you, can say you were there for the advent of the Apple iPod.”
The writer almost immediately interrupts the sentence to emphasize the importance of the reader’s qualification to make the claim. The interruption, however, is weak because it is accomplished with a pair of quotidian commas rather than two dashing dashes.
Also, note that if the writer had correctly punctuated the parenthetical phrase (“yes, you”), the result would be a confusing sequence of three commas (“You, yes, you, can say . . .”), which would further diminish the impact of the interruption. The correct treatment is “You — yes, you — can say you were there for the advent of the Apple iPod.” (The third parenthetical option, to place “yes, you” in parentheses, is the equivalent of whispering the phrase, which is not the connotation the writer intends.)
3. “The potato, and for that matter ginger root, are not true roots, but underground stems.”
This sentence is a more complicated variation of the one in the previous example — complicated, because the interjection (“and for that matter ginger root”) itself includes a parenthetical phrase that the author has erred in not setting off with punctuation: “For that matter” is an interjection within the phrase “and ginger root.”
The larger interjection should be set off by em dashes, though parentheses are also correct; commas will suffice for the one within: “The potato — and, for that matter, ginger root — is not a true root, but an underground stem.” (Note, too, that I altered the sentence’s plural construction to a singular one: Factually, ginger root is also an underground stem rather than a true root, but in the sentence as it is structured, because ginger root is within a parenthetical phrase, is and the nouns root and stem refer only to potato.)