The model captures all the loans that qualify for a particular pool segment as of a particular point in time to form a cohort, then tracks the cohort over the remaining lives of the loans to determine their behavior. Loans are pooled by similar characteristics.
The Cohort method is simple and straightforward, but still sophisticated enough that it is being adopted by many large institutions. In essence, you’re taking a snapshot at a particular point in time and rolling it forward over a period when enough of your portfolio has had enough experience to determine a loss rate. You determine where to set that bar – for example, when 90 percent of loans have been exhausted – and compile those years’ loss rates for a historical average. Then you layer over with qualitative factors for your future expected losses. Prepays are implicitly considered.
Advantages of Cohort Methodology
- Requires less data to calculate than some other methods
- Are less prone to spikes in losses in a particular year
- Can be run on pools with broader risk characteristics than some other methods
Disadvantages of Cohort Methodology
- Difficult to isolate losses expected in early or older years of a loan
- More difficult than some other methods to revert to mean for a non-forecastable period
- Could require more qualitative assumptions than others