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COMPAS and Bias in Recidivism Algorithms

COMPAS (Correctional Offender Management Profiling for Alternative Sanctions) is a commercial risk assessment tool from Northpointe, now Equivant, that scores how likely a defendant is to reoffend.

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最終更新日 2026年9月15日

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概要
ディープダイブ
戦略的影響
The Future of COMPAS and Bias in Recidivism Algorithms
現実世界の実装
リスクとガードレール
実装ロードマップ
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よくある質問

概要

It became the central case study in algorithmic fairness after a 2016 ProPublica investigation found that its errors fell unevenly by race. The debate that followed showed that common definitions of fairness can mathematically conflict.

ディープダイブ

In May 2016, ProPublica published "Machine Bias." It analyzed COMPAS scores for roughly 7,000 people arrested in Broward County, Florida, in 2013 and 2014, and checked who was charged with a new crime within two years. Race is not an input to COMPAS, which uses criminal history and a questionnaire. Even so, ProPublica found that among people who did not reoffend, Black defendants were nearly twice as likely as white defendants to have been labeled higher risk. Among people who did reoffend, white defendants were more often labeled lower risk. Put simply, false positive rates were higher for Black defendants and false negative rates were higher for white defendants. ProPublica also reported that overall accuracy was modest, around 60 percent. Northpointe replied that the tool was fair by a different standard. At each score level, Black and white defendants reoffended at similar rates, so a given score meant the same thing regardless of race. This property is called calibration, or predictive parity. Each side's numbers were largely correct. Researchers then showed why both could be right. Jon Kleinberg, Sendhil Mullainathan and Manish Raghavan, and separately Alexandra Chouldechova, proved that when two groups have different base rates of the outcome, a score cannot be calibrated and also have equal false positive and false negative rates across groups. The only exception is a perfect predictor. So choosing a fairness definition is a value judgment, not a technical detail. In State v. Loomis (2016), the Wisconsin Supreme Court allowed judges to consider COMPAS at sentencing. It ruled that the score could not be the deciding factor and required written cautions to accompany it. A 2018 Dartmouth study by Julia Dressel and Hany Farid found that untrained online volunteers predicted rearrest about as accurately as COMPAS. A common misconception is that leaving race out removes bias. Correlated inputs such as prior arrests can carry historical policing patterns into the score.

技術的な洞察

The conflict follows from confusion-matrix arithmetic. Let p be a group's prevalence (base rate), PPV its positive predictive value and FNR its false negative rate. Chouldechova showed that FPR = (p / (1 - p)) x ((1 - PPV) / PPV) x (1 - FNR). If PPV and FNR are held equal across groups, which is what predictive parity plus equal miss rates would require, then a group with a higher p must have a higher FPR. You can equalize some of these quantities, not all. In practice, auditors report several metrics per group with confidence intervals and make the trade-off explicit. Options include group-specific thresholds (legally sensitive), accepting calibration and disclosing error-rate gaps, or changing the decision the score feeds so that a false positive does less harm, for example a reminder call instead of detention. Label choice also matters: rearrest measures police contact, not offending, so differences in enforcement can bias the ground truth.

戦略的影響

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The Future of COMPAS and Bias in Recidivism Algorithms

Risk assessment tools are still used in many US pretrial and sentencing settings, and the debate continues. Some reform groups back them as more consistent than unaided judgment, while others argue they entrench historical bias. Likely developments include requirements for local validation, public reporting of error rates by group, and access for defendants to how scores are produced, though the pace varies by jurisdiction. The impossibility result is now a standard part of fairness teaching and shapes audits in hiring, lending and healthcare. It does not settle which definition to use. That remains a policy choice about which errors a society is more willing to accept.

現実世界の実装

In a county that uses COMPAS, a pretrial officer combines a defendant's questionnaire answers with their criminal history. The result is a set of decile scores from 1 to 10 that appears in the report sent to the judge.

After State v. Loomis, a Wisconsin judge who receives a COMPAS report also receives written cautions. They warn that the method is proprietary, that the scores describe groups rather than individuals, and that questions have been raised about racial disparities.

A data scientist auditing a lending model computes both calibration and false positive rates for each group. As with COMPAS, equalizing one breaks the other because the groups' base rates differ.

Researchers following the 2018 Dartmouth study show that a simple model using only age and number of prior convictions predicts rearrest about as accurately as the full COMPAS score.

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よくある質問

What is COMPAS and Bias in Recidivism Algorithms?

COMPAS (Correctional Offender Management Profiling for Alternative Sanctions) is a commercial risk assessment tool from Northpointe, now Equivant, that scores how likely a defendant is to reoffend. It became the central case study in algorithmic fairness after a 2016 ProPublica investigation found that its errors fell unevenly by race. The debate that followed showed that common definitions of fairness can mathematically conflict.

Among defendants who did NOT reoffend, what did ProPublica find about Black defendants compared with white defendants?

ProPublica found a higher false positive rate for Black defendants. Among people who did not reoffend, Black defendants were nearly twice as likely to have been labeled higher risk.

What was the core of Northpointe's defense of COMPAS?

Northpointe argued predictive parity, also called calibration: a given score corresponds to a similar reoffense rate regardless of race.

Under what condition does the impossibility result say calibration and equal error rates cannot all hold?

Kleinberg, Mullainathan and Raghavan, and separately Chouldechova, showed the conflict arises whenever base rates differ and prediction is imperfect.

What did the Wisconsin Supreme Court hold in State v. Loomis?

The court allowed COMPAS to be used alongside other factors, with written cautions about its limits, but ruled it could not be the deciding factor.

Is race an input to COMPAS?

Race is not an input. Disparities can still arise through correlated inputs and through differences in base rates.

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