求翻译：现场调查是研究外源性化学物对接触人群健康影响的主要方法，对确定外源性毒物的有害效应极为重要。(1)描述性流行病学调查：提出病因假设，已知病因了解严重程度( 2)分析性流行病学调查：验证假说，确定因果关系。是什么意思？

Site survey is the study of exogenous chemicals on primary method to contact health effects, is extremely important to determine the harmful effects of exogenous toxins. (1) descriptive Epidemiology: make etiological assumptions, understand the severity of known etiology (2) analytical Epidemiology: Verify hypotheses, determining causation.