Anytime stock market studies are done the results or the outcome are plotted on a long term chart of usually the Dow Jones Industrial Average or the SP500. It might be a breadth study, sentiment, average intra-year draw-downs, average yearly gain, etc...Any and all negative studies plotted on a long term chart will look as a non-event, every correction will also look like a non-event, and because you are looking at history and know what the end result is that will give you a false sense of security that you can withstand any and all corrections and everything should be disregarded. This is the furthest thing from the truth. "Everyone has a plan until they get punched in the mouth"
But more importantly, I believe that plotting the outcome of negative or positive studies on a long term chart of the DOW and or the SP500 is a little misleading. If your entire portfolio consists of only the DOW JONES equivalent ETF and or the SPY then you can take these studies to the bank. However, if like most investors your portfolio consist of individual stocks then these studies can be extremely harmful to your portfolio. It can give you a sense of complacency that since the "market" has shrugged off every major negative event overtime that your stocks will do the same thing as well. But most stocks unlike the major indicies don't come back, most go out of business, or show a negative return over their lifetime.
“The Russell 3000 index measures the performance of the largest 3000 U.S. companies, 98% of the investable U.S. equity market. 40% of the stocks had a negative return over their lifetime, 20% of stocks lost nearly all of their value, 10% of stocks recorded huge wins over 500%. 80% of the gains are a function of 20% of the stocks. --The Ivy Portfolio
If time is on your side then you can turn a blind eye to every major negative event and use them as a buying opportunity if your entire portfolio consist of major index etf's; SPY, DIA, QQQ, IWM. But stay extremely vigilant with individual stocks and don't fall for these studies if your portfolio mainly consist of stocks. Remember the nifty 50, Tandy, Dell, CSCO, AOL, the 3D stocks, and pretty much every stock that was a leader at some point in their lifetime, a majority if not all never roared back like the Dow and SP500 have done after corrections, major events, or the 2009 to present huge wall of worry. Don't be fooled.