If the sense is of one unit, viewed as a single entity, then the noun should generally be treated as singular.
Despite this commonsensical advice, found in most style guides, the idea that none must always be treated as singular has persisted.
If a plural does not end in S, you add apostrophe+S: children's games, people's republic, women's rights, etc. Phrases such as butcher's hook, collector's item, cow's milk, goat's cheese and writer's cramp are best treated as singular.
Sums of money are usually treated as singular because the focus is on the sum, not on individual bills or coins: Ten dollars buys less now than five did then.
Plural nouns that do not end in S take an apostrophe and S in the possessive: children's games, old folk's home, people's republic, etc. Phrases such as butcher's knife, collector's item, cow's milk, goat's cheese, pig's blood, hangman's noose, writer's cramp, etc are treated as singular.
"We have reached a critical mass where we can no longer be ignored or treated as singular issues," said Annie Clark, alumna of UNC-Chapel Hill, one of five women to file a complaint against her former university.
Honesty overused — treated as a singular virtue by itself — can actually lead to cruelty.
Every time a case like this comes out, it's treated as a singular event.
When the idea is one entity rather than two people, couple may be treated as a singular: Each couple was asked to give $10; The couple was the richest on the block.
Yet our enumeration and understanding of the many subtypes of lymphoma have been relatively slow, perhaps in part because for many years lymphoma was treated as a singular entity rather than a group of distinct diseases.
In a sharp-interface model developed recently by Hochrainer and El-Azab (2014; 2015), the void surface is treated as a singular surface across which jump conditions are applied.
