As a tool for measuring risk to the public, the Sex Offenders Register alone is an imprecise guide.
Roy Whiting was jailed for the murder of Sarah Payne in 2000
The register contains the details of all those convicted of a crime in England and Wales under the 2003 Sexual Offences Act.
A predatory paedophile such as Roy Whiting, the murderer of Sarah Payne, would be on it.
And so would a 16-year-old convicted of having sex with a child of 15.
Someone who has merely received a caution - such as the Norfolk teacher, Paul Reeve - would also be placed on it.
But, for the police, this is simply a basic building block of information.
Thanks to a new intelligence database, called ViSOR, police forces have access to much more information which will give them a better chance of assessing the risk to society posed by an offender.
ViSOR, which stands for Violent and Sex Offenders Register, tells police officers how many registered sex offenders are in their area and for what crimes they have been placed on the register.
The requirements for sex offender registration have been tightened since the register was set up under the 1997 Sex Offenders Act.
Offenders must now register with the police within 72 hours of being convicted or cautioned. They must give their name, date of birth, home address and national insurance number - if applicable.
Initial reporting to a police station has to be in person. The police may apply for anyone convicted of a sex offence abroad to be placed on the register.
It may also be a condition of registration that an offender notify the police if he or she is intending to travel abroad.
There is a sliding scale applied to offenders required to go on the register.
Anyone getting a jail term of 30 months to life is subject to an indefinite term of registration - usually for the rest of their life.
A sentence of six months to 30 months is accompanied by 10 years on the register.
A sentence of under six months will require registration of up to seven years.
That includes those cautioned or given a community rehabilitation order.
For those under 18, the length of time on the register is usually half that of the adult term.
Failure to register attracts a fine and or a jail sentence of up to five years.
According to the Home Office, the compliance rate is around 97%.