Measuring the number and gravity of cases of actual torture is extremely difficult, for apparent reasons. It takes place in secret, and the people subjected to torture are often in prison long afterwards, or don’t survive it. Either way, they can’t tell us.
That’s why people try to find other ways to measure torture. Asking the public when and under which circumstances they think torture is acceptable may give an approximation of the likelihood of torture, at least as long as we assume that in democratic countries governments will only engage in torture if there’s some level of public support for it. This approach won’t work in dictatorships, obviously, since public opinion in a dictatorship is often completely irrelevant.
However, measuring public opinion on torture has proven to be very difficult and misleading:
Many journalists and politicians believe that during the Bush administration, a majority of Americans supported torture if they were assured that it would prevent a terrorist attack. … But this view was a misperception … we show here that a majority of Americans were opposed to torture throughout the Bush presidency…even when respondents were asked about an imminent terrorist attack, even when enhanced interrogation techniques were not called torture, and even when Americans were assured that torture would work to get crucial information. Opposition to torture remained stable and consistent during the entire Bush presidency.
Gronke et al. attribute confusion of beliefs [among many journalists] to the so-called false consensus effect studied by cognitive psychologists, in which people tend to assume that others agree with them. For example: The 30% who say that torture can “sometimes” be justified believe that 62% of Americans do as well. (source)