Sir, see this question,pls.

people(who're) involved, people (who're) concerned ======= related people?

I don't know when i can eliminate (who're) in my essays.

Eg: There is a beautiful girl (who is) flirted by someone. Or ......girl (who is) regareded as a hot chick.

They are just what i take for example. I'm always not sure how to eliminate (who, which, what, etc. as well as (is, are, be))

And there is another question:

eg. Female smokers saw the opposite trend. They increased from 16% in 1960 to 24% in 2000.

Can I change it to " .......trend, who increased .....?

Sometimes I'm at a lost about such organization of sentences, because I don't know their differences except one sentence or two sentences.

Pls tell me know. Thank you.