Keeping score is important. If you are trying to lose a few extra pounds, stepping on the scale from time to time allows you to evaluate your performance. In school, teachers assign grades to assess students' level of understanding. In baseball, statistics on nearly every aspect of the game are used to determine the pitchers and the batting order. Data and statistics are useful for making informed decisions. During World War II, the United States government wanted to better understand the economy's ability to generate the necessary materials for the war effort. This led to the development of gross domestic product (GDP) as a means of measuring economic output. GDP is important today as an overall indicator of economic performance.