Historians need many kinds of evidence before they are able to reconstruct the past. Like a detective, the historian looks for evidence in the things people have left behind. An historian begins research by asking questions. Once the historian has decided which questions to ask, he or she begins to collect evidence.
The evidence that an historian uses comes in many different forms. Secondary evidence, which is secondhand information, is sometimes used but more often, an historian choses to use primary resources: these include official records, such as graduation or marriage certificates; public records, such as newspapers; and private documents, such as letters and diaries. Historians also use photographs, paintings, drawings, artifacts, architecture, and oral history. It is important that historians use a variety of evidence so that they get as complete a picture of the topic as possible.
Once historians have collected all their evidence they check to see if they have enough sources, if the sources are correct, and if the sources conflict with each other. It is very important for historians to check their sources. If they make mistakes, and other historians base their new work on these mistakes, incorrect stories will be told to the public.