Find standard deviation between two series

I have stock price histories for an index and an ETF that tracks the index.  I'd like to find how well the ETF is actually tracking the index.   What is the best way to do this - percentage, standard deviation?  There are also ETFs that track in the opposite direction and I'd like to do the same for them.
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rebase the values at the start of the period you are reviewing to 100 by multiplying by a constant factor to both the values, then for subsequent values apply the same factors to both values to see what the divergence is.

so if at start dow jones = 9156     then factor1 = 100/9156
                      ETF = 87.8                           factor2 = 100/87.8

starting  rebased values would be dow jones = 100/9156 * 9156 = 100
ETF = 87.8 * 100 / 87.8 = 100

then if today dow jones = 8145, then rebased value = 8145 * 100/9156 = 88.96

if say etf = 80.0. then rebased value = 80 * 100 / 87.8 = 91.11

so in that ficticious case it would have perfromed at 91.11/88.96 = 102.4% of market, or 2.4% over the market

just an idea.

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