De klokvormige curve

A bell curve, or normal distribution, is a symmetrical, bell-shaped graph showing how data points are distributed around the mean.

Many human traits, such as height, naturally follow the bell curve distribution.

Theoretically, the bell curve is mathematically defined and smooth, reaching its peak at the center. It tapers symmetrically on both sides, approaching the X-axis without ever touching it.

For example, most intelligence tests have a mean score of 100, with each standard deviation or SD equalling 15 points, showing how scores are spread across the population.

A score of 85 is one SD below the mean, and 115 is one SD above. Scores ranging from 85 and 115 are considered average, including about 68% of the population.

Approximately 2.2% of the population scores over two SDs above the mean, 130 or higher, indicating superior intelligence.

Conversely, around 2.2% of the population scores more than two SDs below the mean, 70 or lower, which may indicate an intellectual disability.