“Misunderstanding of Statistics Confounds Analyses of Criminal Justice Issues in Baltimore and Voter ID Issues in Texas and North Carolina”

James Scanlan at Fed Soc blog:

I have written here before, most recently in “Things the President Doesn’t Know About Racial Disparities”  (Aug. 5, 2016), about the way the federal government, including the President, base many civil rights law enforcement policies on an understanding of statistics that is the exact opposite of reality. In particular, policies involving fair lending, school discipline, and criminal justice are based on the belief that relaxing standards or otherwise reducing the frequency of adverse outcomes will tend to reduce (a) percentage differences in rates of experiencing the outcomes and (b) the proportion racial minorities and other disadvantaged groups make up of persons experiencing the outcomes. In fact, however, reducing the frequency of an outcome tends to increase both (a) and (b).

I illustrate the pertinent statistical pattern by showing how lowering a test cutoff, while tending to reduce percentage differences in pass rates, tends to increase percentage differences in failure rates. Lowering a cutoff will also tend to increase the proportion the lower-scoring group makes up of both persons who pass the test and persons who fail the test.

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