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Historical classification groups the various history topics into different categories according to subject matter as shown below.
See also Periodization.
Although there is arguably some intrinsic bias in history studies (with national bias perhaps being the most significant), history can also be studied from ideological perspectives, which practitioners feel are often ignored, such as:
A form of historical speculation known commonly as counterfactual history has also been adopted by some historians as a means of assessing and exploring the possible outcomes if certain events had not occurred or had occurred in a different way. This is somewhat similar to the alternate history genre in fiction.
Lists of false or dubious historical resources and historical myths that were once popular and widespread, or have become so, have also been prepared.