I rediscovered a "toy" today, a correlation tracker at:
Input two stock or ETF symbols and it gives a correlation between the two. For example enter VTI (total U.S. stock market) in the top slot and SMH (semi-conductor ETF) in the lower, then select one year, and the correlation is 0.94. This means a very high correlation (1.00 is the highest), and that a person might only be getting a -6% to +6% difference by trading SMH vs. VTI.
At first glance, that 6% seems small given how narrow and at times volatile the semi-conductor industry is. It can be an interesting tool or toy for those that enjoy playing with numbers and symbols. Option traders tend to like playing with numbers, that's one reason they choose to trade options. Enjoy.