Balance and mean

Use the diagram to explain that the sample mean is where the axis would balance if each data point was a solid weight.

The second and third data sets are skew. Use them to explain the high 'leverage' and disproportionate influence of the extreme values on the sample mean. (Note that only 7 of the 20 values for the Sunshine Hours data set are less than the mean.)

The first data set gives the times taken to answer 20 technical support queries about an accounting package.


Solar cookers are potentially a cheap and environmentally friendly alternative to wood in the developing world. As part of a study of their potential in Botswana, data were collected on the number of sunshine hours in Gaborone. The second data set gives the total sunshine hours on 25th February each year from 1978 to 1997.


The last data set gives the survival times (in days) of 72 guinea pigs that were injected with tubercle bacilli.