Positive Predictive Values

Experimentation
Tool
Bayesian methods
A small webapp that let’s you play with base rates, true positive, and false positive results to see how the math of Bayes Theorem corrects for the Base Rate Fallacy and our broken intuition around conditional probabilities.
Published

June 18, 2026

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Experimentation · Tool · Bayesian methods Live

Positive Predictive Values

A small webapp that let’s you play with base rates, true positive, and false positive results to see how the math of Bayes Theorem corrects for the Base Rate Fallacy and our broken intuition around conditional probabilities.

Open the app ↗
Python Marimo
test sensitivity · 95% 1% base rate → 8.8% PPV 0% 5% 10% 15% 20% base rate (prevalence) 0% 25% 50% 75% 100% P(D | +) · BASE-RATE FALLACY positive predictive value vs. prevalence fig — the base-rate fallacy in one curve sens 95% · spec 90%

What it does

Learning about the base rate fallacy and how Bayes Theorem resolves our broken intuition around conditional probabilities was my first big “A-HA!” moment in statistics. I’ve been hooked ever since.

This simple webapp let’s you play with population, prevelance, sensitivity, and specificity to see how the values ripple through Bayes Theorem to a rigorous and sound posterior positive predictive value.

For a detailed refresher on this topic, see my post on Positive Predictive Values.