A persistent world can be achieved by developing and maintaining a single or dynamic instance state of the game world that is shared and viewed by all players around the clock. The persistence of a world can be subdivided into "game persistence", "world persistence" and "data persistence". Data persistence ensures that any world data is not lost in the event of computer system failure. World persistence means the world continues to exist and is available to players when they want to access it. Game persistence refers to the persistence of game events within the world (a Groundhog Day MUD is a virtual world where the entire (game) world is reset periodically). When referring to a "persistent world", world and game persistence are sometimes used interchangeably. The persistence criterion is the trait that separates virtual worlds from other types of video games.