Key ideas

• In a normal distribution, most scores cluster near the middle and fewer fall at the extremes, and the mean, median, and mode all fall at the center.
• In a skewed distribution, the mean is pulled toward the tail, the mode stays at the highest peak, and the median falls between the mode and the mean.
• Income is positively skewed: a few very high earners pull the mean up, so mean income is higher than median income.
• A bimodal distribution has two peaks, which often suggests the data come from two different groups, such as younger and older participants.

Key terms

histogram

A graph that groups similar scores into bars; each bar is a range of scores, and its height shows how many fall in it.

normal distribution

A symmetrical, bell-shaped distribution where most scores are near the middle and few are at the extremes; also called a bell curve.

positively skewed distribution

A distribution with a long tail toward the higher values, created by a few unusually high scores.

negatively skewed distribution

A distribution with a long tail toward the lower values, created by a few unusually low scores.

bimodal distribution

A distribution with two peaks, or two modes (bi means two); often suggests two different groups in the data.

Watch out