The method used to collect data can have a significant affect on what the results seem to show. A good illustration of this is in British crime statistics.

There are two rival sets of statistics representing crime in Britain: the British Crime Survey and the police’s own figures. Neither is without difficulties.

The British Crime Survey has some clear limitations. Because it is based on asking members of the public about crimes in which they have been the victim, it has blind-spots. Victims of murders, for example, cannot respond to surveys, and so do not show up in the figures. Crimes committed against institutions (e.g. businesses), rather than individuals, are also omitted. Victimless crimes, such as drug use are overlooked by the survey too. The method in which the data is collected thus distorts the statistics, restricting their value.

The police figures are, in many ways, just as unsatisfactory. They only reflect reported crime, meaning that only a fraction of the more minor crimes committed make it in. This also means that any change in people’s willingness to report crimes will be indistinguishable from a change in the number of crimes committed; how are we to tell whether crime is falling, or confidence in the police is falling? Add to this the fact that the way in which crimes are classified has changed over time, and it becomes clear that the numbers can be misleading.

For these reasons and more, crime statistics cannot be taken at face value. Interpreting the figures is a complicated matter.

In October 1999, Warren Blackwell was convicted of indecent assault. He had, it had been alleged, raped a woman at knife-point on New Year’s Eve. He served more than 3 years in prison for this alleged crime. The conviction was overturned, well after his release, this month.

Mr Blackwell’s conviction was quashed because the credibility of his accuser was undermined. His initial conviction rested in large part on the testimony of his alleged victim. Once an investigation by the Criminal Cases Review Commission had damaged her reputation, her testimony was no longer deemed sufficient grounds for a safe conviction. The verdict on Blackwell was changed, belatedly, to ‘not guilty’.

The accuser, it turned out, had a history of making rape allegations. She had previously accused at least five men of rape, but never before had her claims been substantiated.

It would be dangerous to argue from the mere fact that a woman has alleged rape on a number of occasions without her alleged attackers ever being prosecuted to the conclusion that she is a serial liar. The conviction rate for rape is only 5%, and this is plausibly due to difficulties in proving guilt beyond a reasonable doubt rather than because most allegations are spurious. A woman might genuinely be raped several times without getting justice.

However, in this case there was enough to call the accuser’s credibility into question. Not only had she made an unusual number of accusations in the past, but at least one of those accusations had been conclusively disproven. She had a history of making false allegations, a track-record of lying.

This case illustrates the importance of reputation in establishing a witness’s credibility. If someone is known to have lied in the past then their credibility is reduced, particular regarding similar claims in similar situations. Once the facts about Blackwell’s accuser were revealed, the case against him was substantially weakened, and he was rightly treated as a victim of a false accusation rather than as an assailant.