<prompt> Translate the following text to EnglishTatizo la tatu lilikuwa tatizo la tofauti za maeneo, ambalo katikati ya miaka ya 1970 liligunduliwa katika masomo ya kiasi kama chanzo kisichojulikana cha tofauti kubwa katika matokeo ya programu kuliko programu yenyewe.  <response>: A third was the problem of site variation, which in the mid-1970s was identified in quantitative studies as an ill-understood source of greater differences in a programs outcomes than the program itself. 