I think that this is a problem of the usage of adjectives and adverbs (that's why I chose this title):
I have a sentence in my presentation, which clarifies that a procedure uses only observations which are known, so I write:
Uses only known observations.
or do I have to write:
Only known observations are used.
or is the long version necessary:
Uses only observations(,) which are known.
(I'm not sure about the comma)