SPRINT'S MOST PRODUCTIVE CITY IN AMERICA STUDY RANKS SIOUX FALLS, SD #8   The Sprint study published in Jan., 1999 ranked Sioux Falls as the eighth most productive city in America.  The ranking is based on the city's economic productivity.  Criteria used for determining the productivity rating include:  employment rates; growth rates in population and employment; average real per capita income; educational attainment and workforce training; output per worker; business sector diversity; per capita income and earnings growth rate; and air transportation accessibility.