The Combined Paternity Index (CPI) is a ratio of paternity to non-paternity.

Thus a CPI of 123,867 can be verbalized:

• Evidence favors paternity over non-paternity by a factor of 123,867 to one
• A match would occur by chance one in 123,867 times
• Paternity is 123,867 times more likely after DNA testing than before

The Probability of Paternity is a mathematically rigorous way of summarizing all the evidence.

DNA Evidence is stated as the CPI and the non-DNA evidence is included in the prior probability.  Genetic Technologies, Inc. uses a prior probability of 50% which indicates that prior to DNA testing we had no opinion as to the likelihood of paternity.