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I'm writing a scientific paper, I want to use i.e. or that is to clarify a term. Are they both correct or either?

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    'Correct' is a term that tends to be institution-specific. If you're just getting in touch with a fellow mineralogy enthusiast, take your pick. But if you're going to be appraised by a university department, see what their style guide demands. If the institution can pass or fail you, go with their preferences. And when you move to a different university, make sure you switch to their rules (people not under their authority should substitute 'rules' or suggestions here). I know one university that doesn't like periods in ie, eg etc. And another one that doesn't like ie, eg etc. Mar 15, 2014 at 10:14

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As Edwin Ashworth observes in his comment beneath the poster's question, house (or academic) style guides usually express a preference for using either "i.e." or "that is" consistently. If your writing is supposed to adhere to a specific style guide, find out what the guide says about "i.e." versus "that is," and follow its dictates. If you're free to use whichever style you prefer, do so.

I note, however, that a couple of style guides widely used in U.S. publishing express a preference for spelling out "that is"—at least in main text. From Words Into Type, Third Edition (1974):

Latin words and phrases. The following abbreviations may be used in all except literary and formal texts, although each of them has a reasonably short and simple English equivalent that would in many instances be preferable. Note that the modern trend is to set them roman type in text.

[first example:] i.e. (id est), that is

And from Chicago Manual of Style, 15th Edition (2003) under the heading "Scholarly Abbreviations":

15.45 Overview. Abbreviations and symbols such as those listed in the following two paragraphs [the list, which is lengthy, includes "i.e."] rarely appear in running text. They are normally confined to bibliographic references, glossaries, and other scholarly apparatus.

The Oxford Guide to Style (2002) offers a British take on the matter:

3.8 e.g., i.e., etc. ... Although many people employ 'e.g.' and 'i.e.' quite naturally in speech as well as writing, prefer 'for example' and 'that is' in running text. (Since 'e.g.' and 'i.e.' are prone to overuse in text, this convention helps to limit their profusion.) Conversely, adopt 'e.g.' and 'i.e.' within parentheses or notes, since abbreviations are preferred there. A sentence in text cannot begin with 'e.g.' or 'i.e.'; however, a note can, in which case they—exceptionally—remain lower case[.]

The publishing houses I've worked for have unanimously preferred spelling out "that is" to using the abbreviation "i.e." in running or main text. I am not aware of any U.S. publishers that endorse Oxford's distinction between use of "i.e." in running text (discouraged) and use of "i.e." in parentheticals (approved). On the other hand, I have worked with many publishers that follow Oxford's (and Chicago's) lead in freely using "i.e." in footnotes and bibliographical references.

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To answer your question directly: there is no difference in their meaning. And, neither is specifically more formal than the other.

That said, when writing for formal purposes you must conform to the style guide that is suggested by the publication or organization for which you are writing.

If there is no preference set by their style manual, then I recommend picking one and remaining consistent. Internal style consistency is just as important as conforming to an outside style guide. (And, should you change your mind later, it makes the process of a word-processing find & replace much simpler.)

If you wish to avoid the matter altogether, you could always substitute "In other words,".

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