It can therefore be argued that the CPI seeks to capture the perception of all forms of corruption affecting the public sphere. To be sure, more detailed information can be obtained by performing in-depth surveys in individual countries. The CPI is an assessment of the corruption level in 85 countries as perceived by business people, risk analysts, investigative journalists and the general public. The CPI does not seek to reflect perceptions of private sector fraud in countries. Unbiased, hard data on the actual levels of corruption in multiple countries is not available.
International surveys are therefore the most credible means available for compiling a ranking.
It is the judgement of the authors of the CPI that, in an area as complex and controversial as corruption, no single source, or polling method exists as a perfect sampling frame, with large enough country coverage, and a fully convincing methodology to produce comparative assessments. This is why the CPI has adopted the approach of a composite index. This year and henceforth the CPI will be based on all available and credible surveys from the past 3 years. If a survey is not replicated and is more than 3 years old it will no longer be included in the index. A number of additional sources were considered for inclusion in the 1998 CPI, but they had to be rejected.