on an individual basis vs singly

on an individual basis and singly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
on an individual basis	No	No	No	Yes
singly	No	No	No	Yes

As adverbs, on an individual basis and singly are synonyms defined as:

Other synonyms of on an individual basis include individually, one by one, separately, severally.

on an individual basis (adverb)	singly (adverb)
apart from others	one by one; one at a time
	apart from others