TI is a reputed organization with well-tested research methods that can be used in this type of research. You cant use a questionaire and neither u can select a sample to collect data as nobody will tell u the truth in a formal survey. However, if u ask in private after taking a person in confidence, s/he will tell u the truth.
By the way, ask any person on the street about his openion about corruption and u will get similary results as of TI report.

__________________
/// Aur be ghum hain zamanay main muhabat kay siwa ///