A Ministry of Justice internal review has confirmed that its Core Person Record system automatically links highly sensitive justice records without a measured false-positive rate, a measured false-negative rate, or an independently verified ground-truth dataset.
The system operates across a population of more than one million records relating to defendants, prisoners, and people on probation.
The Ministry’s own Data Protection Impact Assessment acknowledges that false links and missed links cannot be eliminated. Nevertheless, the approved assessment records zero identified risks and a residual-risk score of 0.0.
Those findings do not establish that every link is wrong, or even that the overall error rate is necessarily high. They establish something more fundamental: the Ministry of Justice does not know the population-level error rate of an identity-linkage system being used operationally across the justice system.
In June 2026, I submitted a Freedom of Information request concerning the Ministry of Justice’s use of Splink, an open-source probabilistic record-linkage library.
The request focused on the Core Person Record and Probation in Court services. These systems use Splink to identify and link records across court, prison, and probation systems, including HMCTS Common Platform, Libra, NOMIS, and nDelius.
I requested five categories of information:
The original response largely directed me to public source code and documentation.
That did not answer the central question.
Model configuration is not model performance. Source code can show how a system is constructed, but it cannot tell the public how often that system links two different people together or fails to link records belonging to the same person.
I requested an internal review.
On 10 September 2026, the Ministry partially upheld my complaint.
The internal review states:
It also confirms that the Ministry does not hold recorded information establishing the observed false-positive or false-negative rates.
The reason given is that calculating a population error rate would require an independently verified ground-truth dataset showing which records across the source systems genuinely belong to the same person.
The Ministry confirms that no such reference dataset exists for the prison and probation population.
This is the central finding.
The Ministry is automatically linking identity records across operational justice systems, but it has no verified dataset against which it can calculate how often those links are right or wrong.
The internal review says that assurance is instead provided through conservative matching thresholds, human review during model development, clerical review of sampled record pairs, and the ability to correct an erroneous link after it is discovered.
Those measures may reduce risk. They do not measure the system’s population-level accuracy.
The Ministry itself acknowledges that reviewing sampled pairs cannot identify all missed matches or establish a population error rate.
The internal review also clarifies what happens when the model assigns a sufficiently high score to a candidate match:
There is no separate clerical-review threshold for newly created links.