Confidence Intervals

What “95% confident” really means

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• 95% confidence interval for a mean: $\bar x \pm 1.96\,\sigma/\sqrt n$.
• “95%” describes the method: 95% of such intervals contain the truth.
• Halving the width takes four times the sample.

Steps

1. A range, not a number
2. Forty intervals
3. What does 95% mean?
4. Build one
5. More confidence, more width
6. Half the width