  Monthly rainfall data from June to October for 39 years was used to generate
Standardized Precipitation Index (SPI) values based on Gamma distribution for a
low rainfall and a high rainfall district of Andhra Pradesh state, India.
Comparison of SPI, with actual rainfall and rainfall deviation from the mean
indicated that SPI values under-estimate the intensity of dryness/wetness when
the rainfall is very low/very high respectively. As a result, the SPI in the
worst drought years of 2002 and 2006 in the low rainfall district has indicated
only moderate dryness instead of extreme dryness. The range of SPI values of
the high rainfall district indicated better stretching, compared to that of the
low rainfall district. Further, the SPI values of longer time scale (2-, 3- and
4- months) showed an extended range compared to 1-month, but the sensitivity in
drought years has not improved significantly.
  To ascertain whether non normality of SPI is a possible reason, normality
tests were conducted. The Shapiro-Wilk statistic, p-values and absolute value
of the median confirmed normal distribution of SPI in both the districts
whereas cumulative probability distribution of SPI indicated deviation from
normal probability in the lower and upper ranges.
  Therefore, it is suggested that SPI as a stand alone indicator needs to be
interpreted with caution to assess the intensity of drought. Further
investigations should include; sensitivity of SPI to the estimated shape and
scale at lower and upper bounds of gamma and impact of other distributions such
as Pearson III on SPI computation, to complement the above results.
