Revisiting the SPI: Clarifying the Process

The number of applications using the Standardized Precipitation Index (SPI) around the world continues to increase (e.g., Agnew, pp. 6–12 of this newsletter, and Komuscu 1999). However, there are relatively few publications explaining the SPI, and occasional misconceptions about the index have occurred. When the SPI was first developed by McKee et al. (1993, 1995), it was meant to address some of the limitations that exist within the Palmer Drought Index (PDI). These first publications were relatively simple introductions of the SPI to the scientific community, appearing in the Proceedings of the Eighth and Ninth Applied Climatology Conferences, respectively, sponsored by the American Meteorological Society. In both cases, the authors define the SPI as the “difference of precipitation from the mean...divided by the standard deviation.” It is this equation, given by Komuscu (1999) and repeated by Agnew, that causes confusion about the SPI