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This is a tough question because we don't have the same "book of rules" that we used year ago. One of the major reasons for this disparity seems to come from what is printed—and accepted—in novels and magazines. Many years ago while completing my undergraduate work, one of my professors hated the long dash (also known as the em dash). Since then, no professors (and I was an English major) has ever cared.
Some teachers today resist the use of the em dash (the dash that is the width of an "m"), while others allow it without question.
The eNotes' reference page on punctuation points out several of the nuances of the dash:
[The dash] carries more emotional weight than a comma does, but it’s more informal than a colon. The dash can help you make a strong point that tells the reader, “Pay attention here.”
Where parentheses contain secondary information, subordinate to the primary content of the sentence, information between the em dashes is too important to be delegated to a parenthetical comment. See the example below. The information contained between the dashes contains powerful commentary on the subject of the sentence:
His thoughts about women—if he had any thoughts at all—were not the kind expressed in polite company.
Two things being shared here are the subject's attitude towards women: his thoughts that are best left unsaid. However, the reader also gets the sense from the information within the em dashes that the man in the sentence not only has little nice to say about women, but also that there may even be the possibility that he does not waste his time (in his mind) to even think about them. Take out the words in the em dash and the meaning of the sentence is much "tamer."
Parentheses are used to include "minor information."
One might write...
The car in the drive (a deep burgandy) told me someone was visiting my brother.
However, with em dashes, one might write:
The car in the drive—its music so loud it woke the neighbor's baby—made it apparent that someone was visiting my brother.
The parenthesis carry less important information, while the em dashes make it clear that the information they contain is of some note.
In the example provided, I think parenthesis are probably more in order than the em dashes. Your example reads:
Some of the other students, Lula and Esteban, for example, have already begun their third project.
Standing by itself, we don't find any reason that the identities of the students on their third project is important. If the sentence made note that Lula an Esteban were usually the last ones finished on a project, I would put their names in em dashes. However, use of "for example" shows that they are simply names of a few of the students working ahead, and that there are other ones beside. The writer only offers two names to support his/her statement. I would write:
Some of the other students (Lula and Esteban, for example) have already begun their third project.
Though strictly speaking, the use of "for example" makes neither the parentheses nor the em dashes appropriate; the sentence is punctuated correctly in its original form. If "for example" is omitted or moved to a forward position (i.e., for example, Lula and Esteban), the parentheses may correctly be used.
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