From a book I am reading:
"Outsider's impressions have been very useful."
A correct apostrophe would be useful also.
Even if we were talking about one outsider, "outsider's impressions" would be missing something. You'd need to say something like "The outsider'simpressions have been very useful" or "The impressions of an outsider have been very useful."
In any case, "Outsiders' impressions" is correct. Remember how many items you're talking about. Ask yourself if the noun is singular or plural.
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