To be or not to be possessive?

1. Plural noun + Noun

students vision

This is not possible. It is not grammatically correct.

2. Singular noun + Noun

student vision

• The first noun gives information about the second noun.
• student tells us what kind of vision
• The reference to the first noun is impersonal and abstract.
• Usually the first noun is inanimate (not a living thing or a group of living things):
• I can't find my pencil case.

3. Plural noun + possessive + Noun

students' vision

• The first noun tells us who possesses the second noun.
• (the vision that is possessed by students)
• The reference to the first noun is personal and concrete.
• The first noun is nearly always animate (a living thing or group of living things):
• Men's magazines are displayed beyond the reach of children.

Returning to our opening example, meaning 3 would appear to be the most appropriate:

This is partly because the second clause in the sentence - broadens their horizons - contains a possessive, and in order for the sentence to be parallel, both clauses need a possessive.