The correlation between SPX and the DJIA will generally be quite high, because sampling 30 stocks (and generally those with the biggest market caps) gives you a good estimate/representation.

As a result, over short periods of time, the spread between the two will be relatively small.

Over long periods of time however, the spread can be significant. See the following:

1. http://avondaleam.com/dow-jones-vs-sp/

2. http://www.thumbcharts.com/101035/DJIA-vs-S-P-500 (Compare 1, 2, 3 and 5 years)

Yes, the calculation of the Dow Jones is a complete joke, but, funnily enough, it has tracked the fairly sensibly calculated S&P 500 surprisingly closely over time.