## 457: Probabilistic Forecast

A map (if the Y-data are gridded or station) is shown indicating the forecast probability for the category with the highest
probability. If more than one forecast is produced, the forecasts can be cycled using the up and down arrows, or typing in the
year. The probabilities are derived from the best-guess forecast (the regression estimate from the CCA, PCR, MLR model,
including that used in the GCM option) by assuming that the errors in the best-guess forecast will be normally distributed (or,
more strictly, will follow a Student's
*
t
*
distribution), with the variance of the errors defined by the sampling errors
in the regression parameters, and by the variance of the errors in the cross-validated predictions (alternative error variances
can be set using
Options ~ Forecast Settings
). If the Y data have been
transformed
it is assumed that the forecast errors of the transformed data will be
normally distributed. From the best-guess forecast and the assumed error variance and distribution, the probabilities of
exceeding the various thresholds can be calculated.

The map title can be changed by right-clicking on the image and then selecting the Customise ~ Title option. There is some flexibility to adjust the colour scaling on the maps: the colour bar limits can be set by right-clicking on the image and then selecting the Customise ~ Contour Limits option, and/or the number of colours can be set by selecting the Customise ~ Number of Colours option. Further options for customising the graphics, including adjusting the size of the fonts, and the size of the images, plus masking of land or lake areas for the maps are available from Options ~ Graphics .

The map can be saved as a graphics file by right-clicking anywhere in the child window and selecting Export from the pop-up menu. A default name is given to the graphics file, but this name can be changed using the browse button. The image quality for all the graphics formats can be improved by increasing the size of the image (see Options ~ Graphics ~ Graph Scaling ).

The forecast probabilities can be saved from the
**
Forecasts
**
tab in
File ~ Output
Results
.