Unreliable evidence presents a significant risk to achieving a fair trial. Its use can result in innocent people being convicted (a miscarriage of justice). Such convictions may, or may not, be quashed on appeal.
Courts have wide powers to exclude evidence where they believe it would have an adverse impact on fairness. Courts can also direct a jury to be cautious about convicting a person the basis of certain evidence. Where unreliable evidence is identified, prosecutions may fail.
Both miscarriages of justice and failed prosecutions involve significant unnecessary expense and wasted time, inconvenience witnesses, distress victims and reduce public confidence in the system.
In order to reduce the risk that evidence is unreliable (and may therefore be excluded), the police are expected to collect evidence in certain ways.