The normal (or Gaussian) distribution is the bell-shaped curve that shows up everywhere — heights, exam scores, measurement errors. It's symmetric, centered on the mean, and described entirely by mean and standard deviation.
The 68-95-99.7 rule
- About 68% of values fall within ±1 standard deviation of the mean
- About 95% within ±2 standard deviations
- About 99.7% within ±3 standard deviations
Try it
If exam scores are normal with mean 70 and SD 10, what fraction scored between 60 and 80?
Within ±1 SD → about 68%.