Date of first rains in Samaru, Nigeria

Use the diagram to show the influence of an outlier on the mean and standard deviation.

Click High outlier to add a value of 240 to the data set. Note the increase in the mean and even larger increase in the standard deviation.

Click Missing value (coded 999) to show how treating such a value as a number in the data set gives an unreasonable sd. (Discuss the 70-95-100 rule of thumb with this mean and sd).

Similarly examine the influence of a low outlier or a year in which there was no planting rain (coded as zero).

The data set shows the date of the first planting rain in Samaru each year from 1928 to 1983, defined as the first occasion after 1st April when there was more than 20 mm of rain within one or two days.

The dates are recorded as the day number after 1st January (e.g. 1st April = day 92, 1st May = day 122).