the question is already standing above. For those who are not familiar with the word "quartile": https://en.wikipedia.org/wiki/Quartile (it is a statistical parameter) I am looking forward for your answers! Kind Regards
To avoid ascribing a personality to a statistical quantity, I would rephrase the sentence to read,
"The quartiles, the importance of which has been laid out in chapter x, are y."
For additional information on relative pronouns, see: https://www.englishclub.com/grammar/pronouns-relative.htm