IRI Home

CPT Help Home -> How to use CPT -> Customising the Results

Customising the Results

The definitions of the three categories used in CPT can be customised using the Options ~ Tailoring menu item. There are two options: absolute values of the thresholds can be set, or prior probabilities of the outer two categories can be specified. Typically the prior probabilities are set at 33% so that the boundaries between the categories are the terciles of the climatogical distribution, but these probabilities can be reset, thus allowing probabilities for more extreme rainfall events to be calculated, for example. However, it should be noted that errors in estimating the thresholds for extreme events can become large, and so changes in the calculated probabilities may be inaccurate, especially when converted to odds.

As an alternative to defining the thresholds using percentiles of the climatological distribution (eg. terciles), the thresholds can be defined in absolute values (mm for example). The values of the thresholds are assumed to be in the same units as the values of the Y input data. Note that the prior probabilities of the above- and below-normal categories may no longer be equal. If the lower threshold is set to be larger than the upper threshold, the two are swapped without warning. The absolute thresholds can also be expressed as anomalies, standardised anomalies, or, if the zero-bound option is set, as percentages of average. By default the climatological period used to calculate the average, and to define the category boundaries set as percentiles, is the full training period, but this can be adjusted using the Options ~ Climatological Period menu item.

Previous


 
Last modified: