To compute an area avergae, the average anomaly is computed and this is added to the climatology. This usually gives a better estimate than a straight average in the presence of missing data, especially when there is a gradient in the clmatoloy over the area (e.g., Niño3). However, this assumption that the noise amplitude is equal in all grid points can give rise to negative numbers for positive-definite quantities such as precipitation. For this variable an error model proportional to the variability is better but this option has not yet been implemented.