A pension fund manager seeks to measure the sensitivity of the company's pension liabilities to market interest rate changes. The manager determines the present value of the liabilities under three interest rate scenarios: a base rate of $9\mathrm{.}4\%,$ and a $88$ bp increase and decrease in rates. The results of the manager$$s analysis are presented below:

Interest rate PV of Liabilities

$9\mathrm{.}4\%-88$ bp $575$

$9\mathrm{.}4\%475$

$9\mathrm{.}4\%+88$ bp $379$

What is the effective duration of the pension fund's liabilities?