50. A more elaborate method for estimating volatility is the generalized autoregressive conditional heteroskedasticity (GARCH) approach, which gives progressively increasing weight to more recent observations of volatility. The weighting scheme is defined by an estimated decay parameter, analogous to the mean reversion parameter in interest rate models. See Tim Bollerslev, “Generalized Autoregressive Conditional Heteroskedasticity,” Journal of Econometrics, 1986, pp. 307–327.