The SPX is down about -15% this Dec, prompting comparisons to 1931 (when our Margin-Adjusted CAPE was 12) and 1920 (when it was 9). My guess is that we end the month closer to -7%, but that with our MACAPE at 37.5 even after this decline, the market loses 50% from here by 2020. 
... and please, don't quote the current forward P/E and thereby reveal the fact that you don't actually examine the relationship between data and outcomes.
Try this with any alternative valuation measure... MarketCap/GVA isn't some "curve-fitted" valuation measure. It's just a comprehensive, apples-to-apples price/revenue ratio, where we also include estimated foreign revenues. When you buy cash flows, you buy decades of them.
Our Margin-Adjusted CAPE, which is, again, effectively a market-wide price/revenue ratio, is similarly consistent. Year-to-year earnings, even forward earnings, are not effective "sufficient statistics" for a 50-year stream of cash flows (and yes, equity duration is ~50 here).
