Hi Izidine - The average is computed as a weighted average: The sum of the data at the grid location times the size of the grid box in which it lies, divided by the sum of the lengths of the grid boxes. If you look at the documentation for all of the transformation, @AVE, @DIN, etc., you'll see this: See the General Information on transformations for important details about this transformation.That link leads to a section about the details of specifying the region over which transformations are to be computed. For @AVE, we've added another note (and maybe we should do that for the rest of the transformations, as this question comes up a lot). In particular, note the discussion about specifying the averaging interval in world coordinates, e.g. longitude, meters, time as in var[x=3.4:4.6@AVE] versus specifying the interval using indices, as in var[I=4:12@AVE]. When the interval is expressed in world coordinates, the weighting is done using partial grid boxes at the edges of the interval. If the interval is expressed using indices, the entire grid cells contribute to the weights.For your question, this means that PR_rg[l=1:3@ave] is the average of the data from January 1 through March 31; but PR_rg[T="15-JAN-1981":"15-MAR-1981"@ave] is the average of the partial grid cells from January 15 to March 15. The data in January and March are included in the average, each weighted by half a grid cell. Ansley On 11/19/2011 6:07 AM, Izidine Pinto wrote: Dear ferret users |