MacDougall ym. 1984 https://www.ncbi.nlm.nih.gov/pubmed/6520032 MacDougall ym. 1984 MacDougall ym. 1984 http://www.ncbi.nlm.nih.gov/pubmed/6520032 nghiên cứu https://www.ncbi.nlm.nih.gov/pubmed/6520032 MacDougall ym. 1984