I'd like to use another word instead of "fraction" that emphasise smallness. What can I say in this sentence:
Among the 20,000 members who were invited to the program, only 20 of them participated which is a fraction of the invited members.
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Actually, fraction is an appropriate word choice.
fraction: a very small part or segment of anything; minute portion
The problem, if there is one, is that fraction also means one number divided by another, with no connotation of largeness or smallness.
One way to address that problem, if there is one, would be to use small or tiny fraction, or perhaps better, small or tiny percentage.
Among the 20,000 members who were invited to the program, only 20 of them participated, which is a small fraction (0.001) of the invited members.
Among the 20,000 members who were invited to the program, only 20 of them participated, which is a small percentage (0.1%) of the invited members.
Either way, both fraction and percentage capture that you want to make a relative comparison, i.e., a comparison of the number of actual attendees to the number of potential attendees based on the number of invitations sent out. Either way though, it would appear that for clarity's sake, both would warrant a subjective adjective such as small or tiny.
As explained by Richard, the word "fraction" by itself is ambiguous and doesn't necessarily communicate that the number of members was too small.
I recommend using the word insignificant.
adjective: small or unimportant: not significant
Among the 20,000 members who were invited to the program, only 20 of them participated which is an insignificant portion of the invited members.