Offer your thoughts or opinion on the input financial query or topic using your financial background.

Fundamentals of creating a diversified portfolio based on numbers?
Most of the “recommendations” are just total market allocations.  Within domestic stocks, the performance rotates. Sometimes large cap outperform, sometimes small cap outperform. You can see the chart here (examine year by year):  https://www.google.com/finance?chdnp=1&chdd=1&chds=1&chdv=1&chvs=maximized&chdeh=0&chfdeh=0&chdet=1428692400000&chddm=99646&chls=IntervalBasedLine&cmpto=NYSEARCA:VO;NYSEARCA:VB&cmptdms=0;0&q=NYSEARCA:VV&ntsp=0&ei=_sIqVbHYB4HDrgGA-oGoDA Conventional wisdom is to buy the entire market. If large cap currently make up 80% of the market, you would allocate 80% of domestic stocks to large cap.  Same case with International Stocks (Developed). If Japan and UK make up the largest market internationally, then so be it.  Similar case with domestic bonds, it is usually total bond market allocation in the beginning.  Then there is the question of when you want to withdraw the money. If you are withdrawing in a couple years, you do not want to expose too much to currency risks, thus you would allocate less to international markets. If you are investing for retirement, you will get the total world market.  Then there is the question of risk tolerance. Bonds are somewhat negatively correlated with Stocks. When stock dips by 5% in a month, bonds might go up by 2%. Under normal circumstances they both go upward. Bond/Stock allocation ratio is by age I’m sure you knew that already.  Then there is the case of Modern portfolio theory. There will be slight adjustments to the ETF weights if it is found that adjusting them would give a smaller portfolio variance, while sacrificing small gains. You can try it yourself using Excel solver.  There is a strategy called Sector Rotation. Google it and you will find examples of overweighting the winners periodically. It is difficult to time the rotation, but Healthcare has somehow consistently outperformed. Nonetheless, those “recommendations” you mentioned are likely to be market allocations again.  The “Robo-advisors” list out every asset allocation in detail to make you feel overwhelmed and resort to using their service. In extreme cases, they can even break down the holdings to 2/3/4 digit Standard Industrial Classification codes, or break down the bond duration etc.  Some “Robo-advisors” would suggest you as many ETF as possible to increase trade commissions (if it isn’t commission free). For example, suggesting you to buy VB, VO, VV instead a VTI.