MR 1 Estimating Market Risk Measures: An Introduction and Overview
A risk analyst is reviewing a counterparty's risk report. The report says that a long-only equity position currently worth USD 80 million has a one-year 95% lognormal VaR of USD 21.016 million. The report also says the position's geometric (log) returns are assumed normally distributed with an expected return of 9% per year, but the volatility figure has been blacked out.
The analyst wants to find the volatility the counterparty used. Assume z = 1.645 at 95% confidence. What annual volatility of log returns does the reported VaR imply?
Show answer and detailed solution
Correct answer: C. 24.0%
Here the VaR is given and the volatility is missing, so the lognormal VaR formula has to be rearranged and solved for σ<sub>R</sub>.
Start from the lognormal VaR formula:
VaR = P × (1 − exp(μ<sub>R</sub> − σ<sub>R</sub> × z))
Write the reported VaR as a fraction of the position value:
VaR / P = 21.016 / 80 = 0.26270
Substitute and get the exponential term on its own:
0.26270 = 1 − exp(0.09 − 1.645 × σ<sub>R</sub>)
exp(0.09 − 1.645 × σ<sub>R</sub>) = 1 − 0.26270 = 0.73730
Take the natural log of both sides:
0.09 − 1.645 × σ<sub>R</sub> = ln(0.73730) = −0.30476
Solve for σ<sub>R</sub>:
1.645 × σ<sub>R</sub> = 0.09 + 0.30476 = 0.39476
σ<sub>R</sub> = 0.39476 / 1.645 = 0.2400
The volatility implied by the reported VaR is 24.0%.
Check: run it forward to confirm. VaR = 80 × (1 − exp(0.09 − 1.645 × 0.24)) = 80 × (1 − exp(−0.30480)) = 80 × (1 − 0.73727) = 80 × 0.26273 = USD 21.02 million, which matches the report.
The one step to be careful with is the drift term. The exponent is μ<sub>R</sub> − σ<sub>R</sub> × z, so when you move things around to isolate σ<sub>R</sub>, the +0.09 drift crosses over and adds to the +0.30476. Both end up on the same side. A student who forgets the drift term lands on the wrong volatility.
The forward and backward versions of this formula test different things. Going forward (parameters to VaR) tests whether you know the formula. Going backward (VaR to a parameter) tests whether you understand it well enough to rearrange it, which is the sharper test. The reversal is feasible by hand because the BA II Plus does the ln and the e<sup>x</sup> steps directly: take ln(0.73730) with the LN key, then isolate σ<sub>R</sub>. Get comfortable with those two keystrokes so the rearrangement does not eat time on exam day.
Why the other options are wrong
- A. Uses z = 2.326, the 99% value, instead of z = 1.645 for 95%. Solving 0.39476 / 2.326 = 0.170. The arithmetic is otherwise correct; the error is pulling the wrong confidence level off the z table.
- B. Forgets the drift term. Solves 1.645 × σ<sub>R</sub> = 0.30476 without adding back the 0.09, giving 0.30476 / 1.645 = 0.185. The drift is easy to lose when the formula is rearranged.
- D. Uses z = 1.282, the 90% value, instead of z = 1.645 for 95%. Solving 0.39476 / 1.282 = 0.308. It is the same type of error as A in the other direction: the method is right and z is wrong.