Read the world in data,
one belt at a time.

Before you can reason about data you have to see it. Learn to read a histogram, find the center of a batch of numbers with the average and the median, and measure its spread with the standard deviation — the three numbers that summarize almost any dataset.

1. 🔢
   Level 1 · ▶ NEXT Variables & Distributions ★★★ ◆
2. 📊
   Level 2 Histograms & the Density Scale ★★★ ◆ 🧩 Builds on L01
3. ⚖️
   Level 3 The Average & the Median ★★★ ◆ 🧩 Builds on L02
4. 📏
   Level 4 The Standard Deviation ★★★ ◆◆ 🧩 Builds on L03
5. 🎓 👑
   Level 5 · EXAM Describing Data Exam ★★★ ◆◆ 🧩 Builds on L04

Chance is a number between 0 and 1, and it obeys just two rules — add for 'or', multiply for 'and'. Build probability from the ground up, learn where independence breaks down, and meet the box model: the single picture Freedman uses to reason about every game of chance.

1. 🎲
   Level 6 What Is Chance? ★★★ ◆ 🧩 Builds on L05
2. ➕
   Level 7 Adding & Multiplying Chances ★★★ ◆◆ 🧩 Builds on L06
3. 🔀
   Level 8 Conditional Probability ★★★ ◆◆ 🧩 Builds on L07
4. 📦
   Level 9 The Box Model: Expected Value & SE ★★★ ◆◆◆ 🧩 Builds on L08
5. 🎓 👑
   Level 10 · EXAM Chance Exam ★★★ ◆◆◆ 🧩 Builds on L09

The same bell-shaped curve turns up everywhere — test scores, heights, measurement errors, chance sums. Learn to convert any value to standard units, read areas off the normal curve, and use the 68-95-99.7 rule to answer 'how unusual is this?' at a glance.

1. 📐
   Level 11 Standard Units ★★★ ◆◆ 🧩 Builds on L10
2. 🔔
   Level 12 The Normal Curve & the 68-95-99.7 Rule ★★★ ◆◆ 🧩 Builds on L11
3. 📉
   Level 13 Percentiles & the Normal Table ★★★ ◆◆◆ 🧩 Builds on L12
4. 🎯
   Level 14 The Normal Approximation for Chances ★★★ ◆◆◆ 🧩 Builds on L13
5. 🎓 👑
   Level 15 · EXAM Normal Curve Exam ★★★ ◆◆◆ 🧩 Builds on L14

When two things vary together, how do you measure it — and how do you predict one from the other? Read scatter diagrams, put a number on association with the correlation coefficient, fit the regression line, and learn the traps: outliers, the regression effect, and why correlation is not causation.

1. ✨
   Level 16 Scatter Diagrams & Association ★★★ ◆◆ 🧩 Builds on L15
2. 🔗
   Level 17 The Correlation Coefficient ★★★ ◆◆◆ 🧩 Builds on L16
3. 📈
   Level 18 The Regression Line ★★★ ◆◆◆ 🧩 Builds on L17
4. 🎯
   Level 19 Prediction & RMS Error ★★★ ◆◆◆ 🧩 Builds on L18
5. 🎓 👑
   Level 20 · EXAM Correlation & Regression Exam ★★★ ◆◆◆ 🧩 Builds on L19

This is the whole point. A sample is small and random, the truth is large and fixed — how do you cross the gap? Meet the central limit theorem and the standard error, build confidence intervals, run tests of significance, and finish with the chi-square test. Earn your black belt.

1. 🧪
   Level 21 Samples, Populations & Bias ★★★ ◆◆ 🧩 Builds on L20
2. 🌊
   Level 22 The Central Limit Theorem & Standard Error ★★★ ◆◆◆ 🧩 Builds on L21
3. 🎯
   Level 23 Confidence Intervals ★★★ ◆◆◆ 🧩 Builds on L22
4. ⚖️
   Level 24 Tests of Significance ★★★ ◆◆◆◆ 🧩 Builds on L23
5. 🔎
   Level 25 The Chi-Square Test ★★★ ◆◆◆◆ 🧩 Builds on L24
6. 🥋 👑
   Level 26 · EXAM The Black Belt Exam ★★★ ◆◆◆◆ 🧩 Builds on L25