Hakanson (1980) used eight elements (As, Cd, Cr, Cu, Ni, Pb, Zn and PCB) for PER calculation but don't have all of them. How can we evaluate the PER with numberless elements?
Dear friend, very interesting question. I am interested to your question and will explain in detail soon (similar to my case). A bit hurry for this time, please remind me.
However, the easiest way is searching the related document by typing the keywords into google scholar. You will find some related articles.
If yet to find the articles, do not hesitate to let me know. InsyaALLAH I will help you in detail.
Good luck. Dr Zol Bahri - Universiti Malaysia Perlis