Glossary

First Quartile: Denoted , the first quartile of a set of data is a number having the property that at least one-quarter of the observations are less than or equal to and at least three-quarters of the observations are greater than or equal to .

Interquartile Range: The interquartile range (IQR) is a measure of spread defined as the difference between the third and first quartiles:

Mean: The mean is the average of the data:

Median: The median, denoted , is the "half-way point" of the data in the sense that the same number of observations is greater than or equal to as is less than or equal to it.

Nonresistant: Summary measures that can be greatly changed by one or more outliers.

Outlier: An extremely unrepresentative data point. A box-and-whisker plot identifies any data value less than -1.5*IQP or greater than +1.5*IQP as an outlier.

Standard Deviation: A numerical value given to describe the spread of values of a data set. The larger the standard deviation, the greater the spread. The formula for finding the standard deviation, s, is :

Trimming: k-times trimming consists of removing the k largest and k smallest observations from the data set. The k-times trimmed mean is the mean of the data that remain after the k-times trimming.

Third Quartile: The third quartile, is a number having the property that at least three-quarters of the observations are less than or equal to and at least one-quarter of the observations are greater than or equal to .