Reading The New Market Wizards on my holiday, I came across an interview about the age of a market. A chap had measured the normal length of bull runs in the equity market and used this knowledge to stay out of trades when the market was getting old (i.e. it seemed likely that the run could reverse).
I thought this could be an interesting measure and have created something similar that works for a variety of markets. The bottom chart, for the S&P 500, shows the length of bull (green) and bear (red) markets, as defined as the price move between the first 20-day breakout and the first breakout in the opposite direction. The pattern is surprisingly regular. I have yet to do any testing to see how useful this measure is. As with my other market personality measures, I expect to refine it over time.