I'm reading Vuolteenaho(1999). In this article, the author investigates whether the variation in stock market valuation level is driven by expected future cash-flows or by expected returns. In part V.B and table 5, the author decomposes the variance of log book-to-market ratio into components using GMM. I know the variance decomposition comes from the equation (16) on page 10 of Vuolteenaho(1999). But I can't understand how to estimate this decomposition by generalized method of moments. Could anyone give some detailed information about this decomposition?
I know GMM is a method of parameter estimation, and I know how to decompose variance using VAR model (like Campbell & Shiller (1988)). I just can't understand how to use GMM to variance decomposion.