Data can be distributed in different ways. One type is called normal distribution. A data set that is normally distributed is symmetric about the mean, $\mu .$ Its graph is a bell-shaped curve and shows the spread of the data in terms of the standard deviation, $\sigma .$

In general, a normally distributed data set has the following properties.

• The area under the curve is $1$ or $100\%.$
• Approximately $68\%$ of the data lies within one standard deviation of the mean.
• Approximately $95\%$ of the data lies within two standard deviations of the mean.
• Approximately $100\%$ of the data lies within three standard deviations of the mean.

The above percents can be divided into individual segments as is shown below.

The area under a normal curve can be analyzed as probabilities or percents to interpret the data set.