The accuracy of this figure, however, is questioned by several experts who argue that the group conducting the surveys only base their data on self-declaration, whereby respondents directly answer questionnaries themselves

여기서 whereby가 해석이 매끄럽게 안되서 질문드려요. whereby를 그것에 의하여 ~ 하는? 이정도로 해석하는걸로 알고 있는데 whereby 이하의 문장이 어디랑 연결되는건지 해석을 잘 못하겠네요;;