Measuring corruption statistically is difficult if not impossible due to the illicit nature of the transaction and imprecise definitions of corruption. While "corruption" indices first appeared in 1995 with the Corruption Perceptions Index CPI, all of these metrics address different proxies for corruption, such as public perceptions of the extent of the problem.

Answer this question, if possible (if impossible, reply "unanswerable"): When did indices on corruption first appear?
1995