Earlier this week, I saw a message in my Twitter feed talking about an unusual correlation pattern between the S&P 500 and the Dow. Unfortunately, the graph produced by the Greg Mankiw recommended site AssetCorrelation.com was grossly inaccurate showing periods of zero correlation between the indices. A few lines of code using the Quantmod package in R can easily replicate the functionality of the site and a prudent analyst can simply load a more reliable data source before arriving at any conclusions. This script takes data from Yahoo finance but you can easily use Google finance or load your own data.
Stephen Rush 