The Majority's Habits Overrule those of the Individual - Batlah Daato Eitzel Kol Adam
General Rules
One cannot prove "normal behavior" from the acts of a shoteh: Shabbat 104b
Whether the practice of an entire country, although other countries differ in practice, still defines "popular method": Shabbat 144b; Eruvin 28a, 29b-30a
In determining which items, when placed on the floor of a Succah Hut, become part of the floor in terms of measuring the height of that Succah Hut: Succah 4a