“A general property of noise is that you can recognize and measure it while knowing nothing about the target or bias. The general property of noise […] is essential for our purposes in this book, because many of our conclusions are drawn from judgments whose true answer is unknown or even unknowable.”

This definition of noise distinguishes it from the more known obstacle to accurate decision making: bias. While bias would seem to have a coherent target, noise reflects the unknowability of the things it is trying to measure. It may therefore be characterized as accidental than deliberate inaccuracy.

“It is not acceptable for similar people, convicted of the same offense, to end up with dramatically different sentences – say, five years in jail for one and probation for another. And yet in many places, something like that happens.”

The authors begin their campaign to convince the reader of the problem of noise with the stark unfairness of people convicted of the same crime receiving radically different punishments. While probation is a warning, a five-year jail sentence takes away the defendant’s freedom for that length of time. This discrepancy in the punishments urges the reader to want to know more about the injustice behind this.

“While few people object to the principle of judicial discretion, almost everyone disapproves of the magnitude of the disparities it produces. System noise, that is, unwanted variability in judgments that should ideally be identical, can create rampant injustice, high economic costs, and errors of many kinds.”

Here, the authors draw the distinction between judicial discretion, which overall is a good thing as it allows highly trained, intelligent individuals to evaluate a case, and system noise, which is a marker of the disparities in sentencing that make punishments more a reflection of the judge than the defendant and their crimes. By referring to the ethics of people in general, the authors appeal to the reader’s common sense and their willingness to understand that system noise is a problem.