What are percentiles?
Percentiles are values that divide a set of observations into 100 equal parts. The percentile rank is the proportion of values in a distribution (of observations) that a specific value is greater than or equal to.
For example, if a stream has an annual average water temperature of 15°C and that temperature is higher than or equal to the temperature of 88 per cent of streams for which you have monitoring results, then that stream is in the 88th percentile.
It is common with water quality statistics to use the 5th, 50th (ie, median) and 95th percentiles to describe a distribution of data. The 5th and 95th percentiles describe the low and high ends of the distribution respectively, while the median describes the value at the middle of the distribution.
For example, see the graph below of annual average water temperature for 100 sites. The 5th percentile describes a value for which only 5 per cent of values (or four values in this case) in the distribution are equal to, or lower. The 95th percentile describes a value for which only 5 per cent of values (or four values in this case) in the distribution are equal to, or higher. The median describes a value for which half of all other values in the distribution are lower and half are higher.
Annual average water temperature for 100 sites
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Last updated: March 2009
