William J. Bernstein, in his most recent book, “The Investor’s Manifesto” writes, “Before diving into the most important issue faced by any investor–the asset allocation decision–you will need to understand four things: save as much as you can, make sure you have enough liquid taxable assets for emergencies,

As requested in a comment, here is the correlation matrix of critical ETFs used in nearly all the ITA portfolios.  I cut the time frame to 23 months of data so VSS and VNQI did not show up with “short-term” data errors.  Identical percentages were assigned to each ETF with