I.e. vs. E.g.

‘I.e.’ and ‘e.g.’ are among the most frequently confused abbreviations in academic and scientific writing, and the confusion is not cosmetic: because the two abbreviations make different logical claims, swapping one for the other can change what a sentence actually says about a method, a criterion, or a dataset.

The core distinction: id est vs. exempli gratia

I.e. abbreviates the Latin id est, ‘that is.’ It restates or clarifies what was just said, and what follows i.e. is understood as a complete equivalent, not one option among several. A useful test: if you can substitute ‘that is’ or ‘in other words’ for i.e. without changing the sentence’s meaning, i.e. is the correct choice.

E.g. abbreviates the Latin exempli gratia, ‘for the sake of example.’ It introduces one or more illustrative examples of a broader category, and the list that follows is understood as partial — other unlisted examples could also apply. A parallel test: if you can substitute ‘for example’ or ‘such as,’ e.g. is correct.

The practical consequence in a manuscript: writing i.e. before an incomplete list implies, incorrectly, that the list is exhaustive, and writing e.g. before a defining criterion understates, incorrectly, that the criterion is optional rather than required.

What style guides (APA, Chicago) actually require

Style guides largely agree on the mechanics, with a few differences worth noting for manuscript preparation:

• Periods: both APA and Chicago style retain periods within each abbreviation (i.e., e.g.) rather than the period-free British convention (ie, eg) sometimes seen outside the US.
• Comma placement: both APA and Chicago call for a comma immediately after the abbreviation (‘e.g., smoking status’ not ‘e.g. smoking status’). Chicago’s guidance additionally treats i.e. and e.g. as normally preceded by an opening parenthesis, a comma, a semicolon, or a dash when they introduce a clause mid-sentence.
• Where they belong in the sentence: APA’s guidance restricts i.e. and e.g. to parenthetical material — inside parentheses only. In the running text of a sentence (outside parentheses), APA style calls for spelling out the equivalent English phrase instead: ‘that is’ rather than i.e., ‘for example’ rather than e.g. This is a common source of manuscripts being flagged in copyediting: i.e./e.g. used directly in prose, unparenthesized.
• Italics: neither i.e. nor e.g. is italicized under current APA or Chicago guidance — both are treated as standard abbreviations rather than foreign-language terms requiring italic treatment, even though the underlying Latin phrases would be italicized if spelled out in full.

The common misuse pattern

The single most frequent error is using i.e. to introduce what is actually a partial list of examples rather than a full restatement — effectively using i.e. where e.g. belongs. This typically happens when a writer is naming illustrative instances of something (a subset of variables, conditions, or categories) but reaches for i.e. out of habit or because it reads as more formal. The fix is the substitution test described above: try reading the sentence with ‘that is’ in place of the abbreviation; if the sentence then overclaims completeness, e.g. is the abbreviation that was actually needed.

A secondary, less consequential error is treating the two as interchangeable filler for ‘in other words’ generally, without checking whether the material that follows is genuinely exhaustive (i.e.) or genuinely just representative (e.g.). When neither condition clearly holds, many style guides and editors recommend avoiding both abbreviations and simply writing the clarification or example out in full English.

Related CASRAI content

For the broader mechanics of introducing and formatting abbreviations in a manuscript, see Research Abbreviations: When and How to Define Them in a Manuscript. For the companion question of voice convention in the same manuscript, see Active Voice vs. Passive Voice. For the broader context this term sits in, see the Scholarly Writing for Researchers pillar.