In nominal relative clauses,the relative pronoun acts like a noun and linking relative pronoun working together. The most common nominal relative pronoun is what, meaning "the thing(s) which". The nominal relative pronoun replaces the noun.
Ex: This is just what we need.
What I want is a bigger computer.
But we use which to refer back to a whole clause rather than the preceding noun only.
Ex: He only did what anyone else would do in that situation, which is panic.
The piano required several men to lift it, which was perhaps not surprising.
The which is replacing the noun (IT), but referring back to a whole clause.
In the paper we wrote:
"(...) children work with family trees, what is done by only a 5% of the respondents."
But instead of using what, we should use which, because it is referring back to the previous clause.
"(...) children work with family trees, which is done by only a 5% of the respondents."
Source: Side, R & Wellman, G (2003) Grammar and Vocabulary for Cambridge Advannced and Proficiency. Longman. England.