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I have a rather technical sentence:

The mean mesiodistal positioning error for the posteriors was 0.09±0.09 mm and 0.05±0.01 mm for the control and experimental groups, respectively.

I feel as though I should be writing 'positioning errors....were' as I am listing two quantities- however, as there really is only one mean (i.e., the number listed), would this really be warranted?

The edit would appear as so:

The mean mesiodistal positioning errors for the posteriors were 0.09±0.09 mm and 0.05±0.01 mm for the control and experimental groups, respectively.

  • Is there two "mean" or one ? – P. O. Aug 31 '16 at 13:55
  • Well, for the control group, there would be one mean i.e., 0.09mm, and for the experimental group, there would be one more mean, i.e., 0.05 – Pete Aug 31 '16 at 14:29
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    There are limits to how much spilled grammar you can mop up with the word respectively. And it helps if it is closer to the action. I'd rearrange the sentence to read "The mean mesiodistal positioning errors for the control and experimental groups' posteriors were 0.09±0.09 mm and 0.05±0.01 mm, respectively." Or "The mean mesiodistal positioning error for each of the control and experimental groups' posteriors was 0.09±0.09 mm and 0.05±0.01 mm, respectively." – Phil Sweet Aug 31 '16 at 14:32
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    I am very tempted to use that expression; however, my editor absolutely hates the use of possessive apostrophes- despite the fact that there is nothing wrong with it. I have to work around it. – Pete Aug 31 '16 at 14:49
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    The question is, how many "mean mesiodistal positioning errors" can the posteriors have? If the answer is "more than one," I don't see any reason to avoid "errors" in your sentence. And if "respectively" is giving you trouble, you can reword as "The mean mesiodistal positioning errors for the posteriors were 0.09±0.09 mm for the control group and 0.05±0.01 mm for the experimental group." In fact, I'd run the singular wording the same way: "The mean mesiodistal positioning error for the posteriors was 0.09±0.09 mm for the control group and 0.05±0.01 mm for the experimental group." – Sven Yargs Aug 31 '16 at 21:14
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Leaving the posteriors and their measurements to one side for the moment, your question is basically asking what the grammatical number should be when using the word respectively.

The following dictionary entry is illustrative, particularly for its examples (emphasis in the examples, mine):

Respectively adverb Separately or individually and in the order already mentioned (used when enumerating two or more items or facts that refer back to a previous statement):

  • they received sentences of one year and eight months respectively
  • These positions are frequently referred to respectively as objectivism and constructionism.

- ODO

As the examples illustrate, the nouns in question should be plural. This is even clearer if we replace respectively with approximations like in that order - e.g. "they received sentences of one year and eight months in that order". There were multiple sentences, so it's written in the plural.

Returning to your example, then, since you are describing multiple errors, you should use "... errors ... were".

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Ignore the phrase "mesiodistal positioning".

The mean error for the posteriors was [...]

Since mean is an aggregate, there is nothing wrong with this ordinarily. Presumably, there is only one mean error for n number of posteriors.

However, because you performed the test for two groups, you do have two mean errors--one for the test group and one for the control group. Therefore it would be correct to use the plural "errors", though I very much doubt anyone would criticize you either way.

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    I have decided to rearrange another sentence with the same issue slightly: The mean angulation error was 1.53±1.57° for the control group and 1.52±1.37° for the experimental group. In this case, would the use of the singular be warranted as the verb is much closer to the noun? – Pete Aug 31 '16 at 14:51
  • Placement of the verb to the noun doesn't really matter at all. Instead, you need to consider how many errors there are. In this case, that will depend on whether you're using the term "mean angulation error" to describe a type of error or an instance of an error. Both contexts could be considered correct for this usage, so don't stress over which you pick. If you decide that it's a type of error, of course there is only one type, so you would use the singular. If you decide that its a reference to the two instances of that kind of error, you may use the plural, since there are two instances. – R Mac Aug 31 '16 at 15:12
  • There is a way to rewrite this sentence without fussing over singular or plural, too. Example: "For the control group, the mean angulation error was 1.53±1.57°, while for the experimental group, it was 1.52±1.37°." In place of the pronoun "it", you could also restate the antecedent instead, as "For the control group, the mean angulation error was 1.53±1.57°, while for the experimental group, the mean degree of error was 1.52±1.37°." – R Mac Aug 31 '16 at 15:15

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