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The rate of infant mortality has declined 16 % in the last 10 years. (correct)

The rate of infant mortality has been declining in the last 10 years. (correct)

The rate of infant mortality has been declining 16% in the last 10 years. (incorrect)

These sentences are from my English grammar book in Korean, so I can't provide the source. Sorry. Could you please explain why the third one is incorrect? Thank you.

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"has been" indicates an action that is still occurring. Therefore, since there is no endpoint the value 16% for a percent change doesn't make mathematical sense.

Using past tense "declined" indicates that the end point is now, so a % change can be calculated.


Additionally if the percent reported was smaller (or a reasonable value) it could be falsely, interpreted as a yearly decrease:

... has been declining 1% in the last 10 years.


To unambiguously indicate an average yearly decrease, something like the following could be used:

... has been declining 1% each year in/for the past 10 years.

My mind prefers to use the preposition "for" but either could work I think.

Additionally, "by" could be inserted before the percent in every sentence. This seems to make it less academic/news-y but conveys exactly the same meaning.

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This is a maths question not an English question

In 10 years the total rate of infant mortality has declined 16 % however the total amount is not equal to the annual rate of decline (rate of decline is accumulative) and that number is not mentioned in this text.

Therefore the generalised statement.

The rate of infant mortality has been declining in the last 10 years is correct

whilst the specific statement of

The rate of infant mortality has been declining 16% in the last 10 years is incorrect.

Also note The rate of infant mortality has declined 16% in the last 10 years is incorrect. It could be corrected by adding by

The rate of infant mortality has declined by 16% in the last 10 years.

by preposition (MEASUREMENT) ​ B2 used to show measurements or amounts: used to say that something increases or changeslink to C.E.D.

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