In reality, scientists do not deal in certainty but in probabilities, and the way they calculate these probabilities is complex. For example, when testifying in court, a fingerprint expert may say that there is a 90% chance of   left the mark, and a one in several billion chance of a match if someone else left it. In general DNA provides information of a higher quality or “individualising potential” than other kinds of evidence, so that experts may be more confident of linking it to a specific individual. But DNA experts still deal in probabilities and not certainties. As a result of all this reality checking, trials are getting longer and more cases that might previously have resulted in quick convictions are now ending in acquittals.