It is important to remember that of the types of evidence commonly used in historical research-documents, maps, archeological finds, anthropological tools (plant life, DNA), social structure and language-only documents and maps can be accurately dated and can be used to directly determine historical facts; the other types can only provide indirect data which, while decisive in supporting documents and maps, in itself can only suggest inferential conclusions.
然而必須要注意的是，歷史研究裡可以使用的五種證據：文獻、地圖、考古遺跡、人類學工具（植披、DNA 等）、社會制度與語言等，只有文獻與地圖兩項，因有確切的年代可考，可以直接判定史實，其餘三種資料都只能參考。