This is a difficult question to answer accurately. Surveys are unreliable because people are not likely to be truthful about behavior that is considered socially unacceptable. Surveys underreport the true percentage.
Even if it is difficult, estimating the percentage of people who engage in a specific illegal or frowned-upon behavior is critical as it has important uses in policy-making and enforcement. Research studies have shown some improvement in accuracy by asking participants to estimate the percentage themselves. People are more likely to be open about what others are doing than they are about themselves. Obviously this works best for populations that are well-known by the participants, such as employees in a company.
Read the full blog post here.