There is no crystal ball that shows how stocks will do and everyone is forewarned that past performance is no guarantee of future performance, but there is a plethora of systems that people are employing.
One very famous prediction that went very awry was made by Irving Fisher, an esteemed economics professor at Yale University in 1929. He stated that “Stocks have reached what looks like a permanently high plateau” just a few days before the great crash of 1929 of the stock market on the eve of the great depression. Not only was his personal wealth compromised, but it also impacted his academic reputation.
As late as Wednesday the 23
, he stood by his educated opinion that the stock values were not inflated. “ For months after the Crash, he continued to assure investors that a recovery was just around the corner. Once the Great Depression was in full force, he did warn that the ongoing drastic deflation was the cause of the disastrous cascading insolvencies then plaguing the American economy because deflation increased the real value of debts fixed in dollar terms.” (http://www.econlib.org/library/Enc/bios/Fisher.html)
As one of the greatest mathematical economists, Irving Fisher clearly explained the central principles of and mathematics underpinnings of his theories. Even with his research and great intellect, he had difficulty translating it into personal wealth.