Various minority groups in England and Wales got a disproportionate fractions of the fines for breaking Covid-19 lockdown rules.

This study strongly suggests that many thugs acted based on bias, but it cannot conclusively prove that, because it does not control for other variables — such as the actions of people who were fined.

For instance, the study found that poor people got a high proportion of the fines. Was this because they had to travel by walking and been seen by thugs, while wealthier people went by car? That would be systemic bias rather than individual prejudice.

Contrast this with the Massachusetts courts' study that showed blacks tended to get bigger sentences than whites for comparable crimes in comparable circumstances. By controlling for the other things that can vary between groups, it pretty much proved that bias was at work in the legal system itself. That showed that the legal system is inescapably to blame for the injustice.