A serial killer is typically a person who murders three or more people, in two or more separate events over a period of time, for primarily psychological reasons. There are gaps of time between the killings, which may range from a few days to months, or many years. This list shows all known serial killers from the 20th century to present day by number of victims, then possible victims, then date. For those from previous centuries, see List of serial killers before 1900. In many cases, the exact number of victims assigned to a serial killer is not known, and even if that person is convicted of a few, there can be the possibility that they killed many more. Organization and ranking of serial killings is made difficult by the complex nature of serial killers and incomplete knowledge of the full extent of many killers' crimes. To address this, multiple categories have been provided in order to more accurately describe the nature of certain serial murders. This is not a reflection of an individual's overall rank, which may or may not vary depending on personal opinion concerning the nature and circumstances of their crimes. The fourth column in the table states the number of victims definitely assigned to that particular serial killer, and thus the table is in order of that figure. The fifth column states the number of possible victims the killer could have murdered. Some of these crimes are unsolved, but are included because they are the work of a serial killer, despite nobody being caught. This list does not include mass murderers, spree killers, war criminals, members of democidal governments, or major political figures, such as Adolf Hitler, Francisco Franco, Hideki Tojo, Suharto, Mao Zedong, Joseph Stalin, or Pol Pot.