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| COMPAS案例中的公平指標選擇與風險課予 |
| Fairness Metric Selection and Risk Imposition in the COMPAS Case |
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| 作者 |
甘偵蓉 |
| Author |
Zhen-Rong Gan |
| 關鍵詞 |
COMPAS、
公平指標、
風險課予、
剩餘錯誤風險、
均等召回率
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| Keywords |
COMPAS; fairness metrics; risk imposition; residual error risk; recall parity
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| 摘要 |
同一演算法採用不同公平指標，可能得到相反評價，因為各指標所比較與限制的群體差異不同。本文以再犯風險評估系統COMPAS 為例，主張在資料、模型、閾值與制度用途固定後，選擇公平指標，就是決定讓哪些群體承擔哪一種改善後仍無法消除的預測錯誤，本文稱為「餘錯誤風險」。這項選擇構成風險課予，至少須滿足兩項證成要求。第一是說明要求，須揭露正類編碼、比較群體、以及所限制與容許的差異。Derek Leben 的均等召回率主張顯示，正類編碼如未被標示，公平指標所限制的錯誤會在錯誤羈押與錯誤釋放之間游移；即使編碼固定，其優先性仍取決於剩餘錯誤風險由誰承擔。第二是對風險承受者的證成要求。本文將Sven Ove Hansson 的風險課予判準轉譯至非互惠的審前羈押情境，主張不會再犯者所承擔的錯誤羈押風險無法由防衛他人的理由證成，導入與使用COMPAS 者須滿足正義與最大平等影響力條件，並向這些被告提供說明、救濟與補償。COMPAS 的現行運作未能滿足這些要求，其導入與使用者因此尚未解除證成負擔。 |
| Abstract |
The same algorithm can be judged fair by one fairness metric and unfair by another, because each metric compares different subgroups and constrains a different kind of disparity between groups. Taking the recidivism risk assessment tool COMPAS as a case study, I argue that once the data, model, decision threshold, and institutional use are fixed, choosing a fairness metric amounts to deciding which groups will bear which kinds of prediction error once all feasible improvements have been made. I call the risk of such errors residual error risk. Because choosing a metric is thus a form of risk imposition, it must satisfy at least two requirements of justification. The first, a disclosure requirement, obliges those who adopt a metric to specify the positive class, the groups being compared, and the disparities the metric constrains or leaves in place. Derek Leben's case for prioritizing recall parity shows why this matters. If the positive class is left unspecified, the error that the metric constrains shifts between wrongful detention and wrongful release. Even once the positive class is fixed, whether recall parity deserves priority depends on who should bear the residual error risk. The second requirement is that the risk be justified to those who bear it. Adapting Sven Ove Hansson's account of justified risk imposition to the non-reciprocal setting of pretrial detention, I contend that the risk of wrongful detention borne by defendants who would not reoffend cannot be justified by appeal to the defense of others. Those who deploy COMPAS must instead meet Hansson's conditions of justice and maximal equal influence, and they owe these defendants explanation, remedy, and compensation. As currently operated, COMPAS falls short on both counts, and those who deploy it have yet to discharge their burden of justification. |
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