Sometimes procedural sources of error (such as contamination from another sample or improper sample preparation) occur, but in many cases, DNA fingerprinting has become more reliable. It would be nice to have more twins (DNA from identical twins will have identical DNA profiles), but of course, that is not practical when determining the connections of people for heredity interpretation or crime cases. Overall, DNA testing is said to be 99.99 percent accurate for most of the tests, thanks to modern technology and the rules for DNA data analysis. In most cases, tests for such things as a crime scene are carried out at least two times from two independent analytical facilities. If a discrepancy occurs between the two tests, then fresh samples are often collected and the entire process is carried out again.