Some surveys focus on all films, while others focus on a particular genre or country. Voting systems differ from poll to poll: some surveys suffer from biases such as self-selection or skewed demographics, while others may be susceptible to forms of interference such as vote-stacking.
Every decade, starting in 1952, the British film magazine Sight & Sound asks an international group of film critics to vote for the greatest film of all time. Since 1992, they have invited directors to vote in a separate poll. 63 critics participated in 1952, 70 critics in 1962, 89 critics in 1972, 122 critics in 1982, 132 critics and 101 directors in 1992, 145 critics and 108 directors in 2002, and 846 critics and 358 directors in 2012.
This poll is regarded as one of the most important "greatest ever film" lists. American critic Roger Ebert described it as "by far the most respected of the countless polls of great movies--the only one most serious movie people take seriously."