Data set
In earlier sections, we summarised aspects of the distribution of values in a data set using measures of centre (e.g. the mean and median) and spread (standard deviation and interquartile range). In this section, we introduce a different kind of statistic that describes other aspects of the distribution.
We mainly use one data set for illustration.
Annual rainfall in Samaru, Nigeria
In most of Africa, the most important climatic variable is rainfall. Rainfall is usually highly seasonal and failure of crops is normally associated with late arrival of rain or low rainfall. A better understanding of the distribution of rainfall can affect the crops that are grown and when they are planted.
The table below shows the annual rainfall in Samaru, Northern Nigeria between 1928 and 1983. (In a later page, we will examine the monthly rainfalls.)
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The total rainfall varies considerably with a minimum of 608.2 mm in 1983 and a maximum of 1481.8 mm in 1954. It is an interesting research question to ask whether there is a decreasing trend in rainfall over these 56 years, but the year-to-year variation is much higher than any such trend, so we will ignore the ordering of the data and simply examine their distribution.
The diagram below shows the annual rainfall data as both a stacked and a jittered dot plot.
Click on crosses to see the exact rainfall and year.