Platinum - Palladium relationship

Ok, so they start with the same letter and are part of the same group of elements, but why are they so often proposed as pairs in trading strategies?  As a spread strategy they look especially tempting right now:

Palladium outshone Platinum for most of 2010.  Is the run over?  Or will it go further?

Cointegration testing

Before attempting to answer this question, perhaps an analysis of the relationship is warranted.  Do PL and PA exhibit co-movement over time?  A chart of the historical prices of both (back-adjusted for contract roll) is as follows:

Cointegration testing in different years leads to the following results:

Year	ADF P value	Hedge ratio	Cointegration half life
2004	0.60	0.30	18
2005	0.94	0.25	33
2006	0.57	0.30	21
2007	0.35	0.27	66
2008	0.29	0.22	18
2009	0.46	0.22	27
2010	0.96	0.33	39

In other words, the ADF test [http://en.wikipedia.org/wiki/Augmented_Dickey%E2%80%93Fuller_test] for cointegration has no significant results in any of the past 7 years.

In a statistical sense there is no reason to assume that PL & PA have any sort of relationship whereby they could be part of a pairs trading strategy.

Looking at the 2010 chart one wonders how many traders have been drawn into this spread strategy waiting for the reversion to arrive…