Why denominators matter in policing bias | Lisa Tompson | TEDxUniversity of Waikato

Quick Overview

Lisa Tompson argues that misinterpreting or selecting the wrong denominator when calculating statistics about policing bias, such as using the general population instead of the population at risk, creates misleading narratives that exacerbate political conflict and undermine efforts for genuine reform.

Key Points: The most dangerous errors in public debate about policing bias stem from using incorrect mathematical denominators rather than false numbers. When calculating police stop or search rates, using the general population as the denominator (e.g., census data) instead of the population truly at risk of being stopped yields misleading disparities. Data showing Māori and Pacific people are stopped nearly four times as likely as white drivers is stark, but the denominator (the population at risk) drastically alters the narrative. In the Northern Ireland Troubles, police service statistics comparing Catholic versus Protestant arrests skewed public perception because the denominator failed to account for the actual populations involved in conflict. Researchers have mapped disparities in over 500 studies across fields like healthcare, climate change, and policing, often finding the same denominator error. When the denominator used is the general population (e.g., census data) instead of the population actually exposed to the risk (e.g., people driving in high-risk areas), disparities appear much larger than they truly are.

Context: Lisa Tompson presents this TEDx talk to expose how fundamental mathematical errors, specifically in selecting the correct denominator when calculating statistics related to policing bias, drive misleading public controversy and hinder constructive policy reform. She emphasizes that while disparities in police stops affecting Māori and Pacific people are real, the way these statistics are framed—often using the entire census population as the baseline—distorts the true level of unfair treatment and fuels political agendas.

Detailed Analysis