Soil strength at Chicago O'Hare International Airport

Use the top slider to explain how jittering (random vertical displacement of crosses) helps to separate dense groups of crosses and therefore describes density better. The Redo jittering button changes the random displacement of crosses.

Mention that the least amount of jittering that separates the crosses is best — moving the slider about half-way for this data set.

The bottom slider changes the display into a stacked dot plot. The slider initially moves the crosses horizontally into classes, then moves them vertically into stacks.

Mention that stacked dot plots are usually better than jittered ones for data display.

Before improvements to runways and taxiways at the Chicago O'Hare International Airport, civil engineers conducted exploration and testing of the area. The data are unconfined compressive strengths (tsf) from a sample of soils.

It can be seen from most displays (but most clearly from the stacked dot plot with small crosses) that there was some rounding of the values — there are peaks at 1, 1.5, 2, 2.5, 3, ... . Some (but not all) of the engineers must have rounded their measurements to the nearest half.