agree how to rebalance is very important

Birkshire Hathaway stock is not truly diversified.  They are hheavily invested in the fields they really understand (or good at).  I uses dow or s&p500 index for comparison as diverdification. I also look back to the my old 401k (company provided) selections.  Some of them are designed for retire at different time (10/20/30 years).  Those funds suppose to have bond/cash/stock diversified and rebalanced each year.  Ten years performance is not very good.