How can an uncountable noun be associated with plural nouns?

Question

I collected data on income in some countries and found that (all) the income data is close to each other.

I am not sure that the expression "is close to each other" in the sentence above I created is correct. I use data as a singular mass noun.

Answer 1

Even if you are using data as a singular mass noun, each other can only be used when there is a plurality of subjects.

Therefore, only the following versions can be correct:

1. The income data is close to itself.
   The income datum is close to itself.

These are grammatical, even if something being "close to itself" is strange. It would likely be rephrased, but the sentence is syntactically sound.

2. The income data are close to each other.
   The points of the income data are close to each other.
   The income data values are close to each other.
   The income datums are closed to each other.

Those are four variations of the plural use. I provided the extra examples as alternatives to using just data.

Note that datums, while obscure, is still a valid plural form of the singular datum. However, since it is obscure, you might want to avoid it.

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Now, consider the following:

My body is long. [singular body]

but

The arms of my body are long. [plural arms]

Finally, look again at the second version of the sentence I used in the plural case:

The points of the income data are close to each other.

It's deliberately vague. You could interpret the income data as either singular or plural—and it wouldn't make any difference to the rest of the sentence, because the plural are goes with the plural points.

This particular construction may best match your need for a singular mass noun interpretation of data while still preserving the idea of it containing, or being spread across, several points.