The great bull market of the 1920s and the spectacular collapse of the New York Stock Exchange (NYSE) in late 1929 occupy a pivotal position in popular explanations of the cause of the Great Depression. In the early part of 1928, the Federal Reserve Board began to feel a little uneasy about the situation in the stock market, where prices had been rising with alarming rapidity. Given that there have been more than 32,000 trading sessions since then, the judgment of at least this swath of history is that in any given six-month period there is a 0.79{bf6128eaee7daf804a40e739f155a69f2d5a72ca2bacccc9954495bcd60bdcac} chance of a daily crash that severe. Historically, this month is the worst month of the year for stocks, and most of the biggest stock market crashes throughout our history have come in the fall. We can infer unfavorable market internals in that instance because we know that cumulative NYSE breadth was …