****Please note: The decile development process is currently being revamped. Posting of deciles will resume after this process has been completed.****

A number of drought/precipitation indices are currently under investigation at CIMH, with the view to them being used together to monitor drought and periods with extremely high precipitation. Deciles is one such index.

Deciles were developed by W. J. Gibbs, and J.V. Maher in 1967. It is a very simple index. Indices are acquired by ranking the rainfall data into deciles (percentiles of ten) so that the lowest 10 percent of the values are in the first decile and the highest 10 percent are in the 10th decile. Half of the data is above the median (the 50th percentile) and half below. The deciles are classified as follows:

Decile classifications
Decile 10:
(highest 10%)
Very much above average
Deciles 8-9:
(next highest 20%)
Above normal
Deciles 4-7:
(middle 40%)
Deciles 2-3:
(next lowest 20%)
Below normal
Decile 1:
(lowest 10%)
Very much below normal

As for the SPI, the deciles are calculated on four different time scales: 1-, 3-, 6- and 12- months, catering for the different types of drought. Here land stations, along with NCEP/NCAR reanalysis data are also used.

Prior to December 2009, the deciles were classified in pairs (1-2, 3-4, 5-6, 7-8, 9-10). The present classification better distinguishes cases closer to extreme and is now used by the Bureau of Meteorology of Australia.