The authors use Vasicek specification to pin down the process that drives the TFP and the interest rate: µ(rt) = κ(γ−rt) and η(rt) = η. The relationship between the risk free rate and the rental rate of capital is given by . The natural problem that arises when estimating this model is that rental rate of capital is difficult to obtain while the risk free rate is readily available, so it’s easier to find the rental rate of capital employing its relation to the risk-less rate than directly estimating it from the data.