Member-only story
Are Homicides up 533% in Portland?
Technically, kind of. But use of this statistic is a classic case of the base rate fallacy.
I’ve long stopped expecting my elected officials to be 100% honest. It’s disheartening, and makes me a cynic for sure, but I’ve accepted that part of their job is to periodically sell their souls and carry the party talking points, even when they know they’re spreading misinformation. I certainly do not respect them for doing this, but whatever. They do it.
This morning I woke up to an email from my representative, Congressman Rob Wittman. He’s an educated man — he has a PhD in a scientific field, and I imagine he knows a thing or two about statistics. Nevertheless, he sent an email parroting GOP talking points and statistics about rising crime rates.
One statistic stood out as particularly outrageous:
“Homicides are up 533% in Portland…”
On first glance, one might assume that Portland has devolved into total anarchy and outright war. But the actual numbers tell a different story. This is where we learn about the base rate fallacy.
What is the base rate fallacy?
In this case, the base rate fallacy, or base rate neglect, is when we are provided with an individual statistic that attempts to presents a strong case. Here, the politicians tell us that there has been a 533% spike in homicides in Portland. This individual statistic, assigned to a certain city (Portland) and events (homicides) has great value in this argument. The argument being, that crime is out of control, and Portland is the epicenter of this crime wave.
The base rate, however, remains neglected. Therefore, we do not even know from where this spike starts.
Why is the base rate important?
When you calculate an increase (or decrease) in anything, you have to start with the original number. This tells us where we were versus where we are now. For example, if you bought a single stock at $10 last year and today it is $12, what is your percent increase in value?
Percent increase = (new value — base value) / base value * 100
