Historical rankings of presidents of the United States

In political studies, since the mid 20th-century, surveys have been conducted in order to construct historical rankings of the success of the presidents of the United States. Ranking systems are usually based on surveys of academic historians and political scientists, or popular opinion. The scholarly rankings focus on presidential achievements, leadership qualities, failures, and faults. Among such scholarly rankings, Abraham Lincoln is most often ranked as the best, while his predecessor James Buchanan is most often ranked as the worst. Popular-opinion polls typically focus on recent or well-known presidents.