How to put an article before a word in a phrase if the first word is uncountable and the second word is countable?

Question

Clinimetrics corner: the many faces of "selection bias"

Now, selection and bias are both nouns. Selection is an uncountable noun and a countable noun. Bias is a countable noun. So, can I put "a" before "selection" on the basis of that it is uncountable? Or I will put it on the basis of the second word, bias? (selection) or the second one (bias). Link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3101070/

Another thing I want to ask is that if there is a noun phrase containg two words, does the selection of an article depend on the first word of the noun phrase?

Answer 1

Selection bias is a compound noun. For compound nouns that are made up only of nouns, only the final noun is ever pluralised, so you can have egg sandwiches, but you can't have eggs sandwich.

The countability of a compound noun is therefore based only on the final noun.

In your example, bias is the final word, and it is normally countable (though, as the link I provided shows, it can also be uncountable), so you can have selection biases or a selection bias.