text: With the system still not accounting for variances in playing difficulty for golfers of different abilities, in 1979 the USGA set to work on how to address the issue with the creation of the Handicap Research Team. The result of their work was the creation of what is now the Slope system. Slope was gradually introduced, firstly in Colorado in 1982, before being implemented nationally from 1987. The USGA then set about making further refinements to the course rating system, which at the time was still largely dependent on length, to take account of many other factors affecting scoring ability for a scratch golfer. The USGA Course and Slope Rating system is now used by most of the world's major handicapping systems.
  The USGA Handicap System is used throughout the jurisdiction of the USGA (i.e. the United States and Mexico), and is also licensed for use in many other countries around the world, e.g. Canada. The USGA has often resorted to the courts to protect the integrity of its handicap system.  In one such case, the California Court of Appeal (First District) summarized the system's history:
  USGA Handicap System overview
  A USGA handicap is calculated with a specific arithmetic formula that approximates how many strokes above or below par a player might be able to play, based on the ten best scores of their last twenty rounds.  The calculation has several variables:  the player's scores from their most recent rounds, and the course and slope ratings from those rounds.