Consumer Reports measured the gas mileage of 38 1978-79 model cars. Other measurements about each car, such as weight and drive ratio, were reported by the manufacturer. A cluster analysis of MPG, Weight, and Drive Ratio for cars reveals three main clusters, which we might identify as large sedans (Ford LTD, Chevrolet Caprice Classic), compact cars (Datsun 210, Chevrolet Chevette), and upscale, but smaller, sedans (BMW 320i, Audi 5000).
The example is effective because common knowledge is sufficient to identify the groups and yet the third group may not be expected before the data are examined.
On statistics packages capable of a three-dimensional, rotating scatterplot, the same three groups can be seen, especially if they are colored or displayed with different symbols. If available, a rotating plot helps to display what the cluster analysis finds.
Note: One observation does not fit well. This is the Buick Estate Wagon (see the related story associated with this datafile).