Stock Market Momentum - Part 1

I'm looking at a systematic, momentum-based stock market strategy.  Why?

• The existence of momentum in markets is proven by numerous academic studies.
• The system I have gives a nice CAGR 14-20%, over 10-15 year periods.
• I will be buying stocks that have gone up.  Completely different from the value stocks I normally buy.
• I want to be able to trade the markets systematically, without having an opinion. 

What is Momentum?

From academic studies, stocks that have gone up in the past 6-12 months are statistically likely to go up in the next 1-2 years ¹.  Most papers use similar methods to measure momentum, for example:

• Value and Momentum Everywhere: Found evidence of momentum worldwide in stock markets, stock indexes, currencies, bonds, and commodities.  For stocks specifically: they selected large, liquid stocks from 4 markets over nearly 40 years.  Stocks' momentum was ranked by their past 12 month return, skipping the latest month.  They were sorted into 3 equal sized groups: Lowest momentum, Medium momentum and Highest momentum.  And re-balanced monthly.  The Highest momentum group outperformed the Lowest by 5.4% (US market), 6% (UK), 8.1% (Europe) and 1.7% (Japan).   
• 212 Years of Price Momentum: Looked at US market from 1801 to 2012.  Same methodology as above.  The Highest Momentum group outperformed the Lowest one by around 4% a year.

Why does Momentum occur?

No one knows.  There are many papers arguing whether its due to 'risk factors' ², or behavior.  I think its behavior.  Humans are herd animals.  Once we see other people doing something, we want to do it too:

• Fads and Fashion.  Whether its primary school children playing, or women comparing handbags, how often have we seen some trend catch on, then grow in popularity, until everyone 'has one'?  At which point they become not cool anymore, and the cycle starts again.  Look at the stock charts of Crocs or Michael Kors for example. 

• Capex Cycle: For mining and heavy industry, it takes years to bring new capacity online, so they respond slowly to increased demand, while the price of their commodity/product shoots up over several years.  At the end of the cycle, prices are sky high and everybody is prospecting/investing.  Leading to oversupply and a crash.  Look at Rio Tinto's stock chart for example: up 7 times from 1999 to 2008, then losing all of that in 2009.

• Success begets success: As business become bigger and stronger, they entrench their position, leading them to become bigger and stronger.  Especially prevalent in technology, with winner-take-all network effects, like with Microsoft or Facebook.  Until someone new comes along, disrupting the market, and the cycle starts again.

Why is it still Profitable?

How can such a simple, dumb, and well known strategy - buying stocks that have gone up - still make money?  Shouldn't everyone be doing it, removing its effects from the market?

Momentum strategies are hard to follow.  They have long and hard drawdowns: a 30% loss every few years is normal.  50-60% losses occasionally occur (the 1929 great depression and the 2000 tech wreck).  And there are long periods...a year or more... of sitting around doing nothing.

There's an interesting presentation by Wes Grey on why Momentum still works.  He considers that, for most fund managers, there is too much career risk in following momentum strategies.  I especially like his 'God Portfolio' (33:00 to 37:00) - not even God can prevent drawdowns!

In the next post I'll look at the system I'm using, and what its like to trade it.


1 Not true for shorter or longer time frames: if we're predicting under one year into the future, or from two to five years, then stock prices tend to mean-revert.↩
2 i.e.: stocks that posses momentum have higher returns to compensate for being riskier.↩