FRM exam 2000 - question 51

fullofquestions

New Member
A portfolio consists of two (long) assets £100 million each. The probability of default over the next year is 10% for the first asset, 20% for the second asset, and the joint probability of default is 3%. Estimate the expected loss on this portfolio due to credit defaults over the next year, assuming 40% recovery rate for both assets.

answer:. £18 million
a) The three loss events are
(i) Default by the first alone, with probability 0.10 − 0.03 = 0.07
(ii) Default by the second, with probability 0.20 − 0.03 = 0.17
(iii) Default by both, with probability 0.03
The respective losses are £100×(1 − 0.4) × 0.07 = 4.2, £100×(1 − 0.4) ×
0.17 = 10.2, £200×(1 − 0.4) × 0.03 = 3.6, for a total expected loss of £18
million.

I cannot recall why you subtract the joint probability from the individual probabilities; the rest is totally clear. Please explain just that one concept. Thanks!
 

jvillatuya

New Member
Hi,

We subtract the joint probability to prevent double counting of events/probabilities. Using a simple example, say there are 2 classes, History and Science, if there are 40 students in History and 50 students in Science and it turns out that there are 10 students who are taking both History and Science classes then we can summarize the breakdown of students as follows:

30 students taking History class exclusively (40 less 10)
40 students taking Science class exclusively (50 less 10)
10 students "JOINTLY" taking the History and Science classes
Total students is 80

We are thereby preventing double-counting.
 
Top