These tests are extremely accurate.  Using the advanced technology for paternity testing, if the DNA of an alleged father and child matches, the probability usually exceeds 99.96% that he is the biological father of the child. 

Should there be mismatches, the alleged father is 100% excluded from being the biological father.  There are no cases of a 20% chance of being the father. 

He is either conclusively shown to be the father, or he is excluded and shown to be 0%.