There is a lot of talk on the net, in the WSJ, etc. about how we're in the longest bull market in recent memory, that "this can't last forever", and so on. Does history suggest that when things finally do turn south, it will be more severe due to the length and scale of the bull market we're in? Or is there no useful, known correlation between the size of a bull market and the proceeding downturn?
Based on A Random Walk Down Wall Street - no. The book's main premise is the market goes up and down randomly, and there is no way to predict future performance based on either historical Data or detailed analysis of the company (Warren Buffet comes up a few times).
Essentially, there is no way for traders to know which companies are embezzling money, about to be hit with tariffs, etc. Because the future is unknown its impossible to predict bull/bear markets as they are caused by future events.