You don't want to own too many funds that are similar, but how do you tell? A high correlation between funds may mean that your portfolio of funds is not as diverse as you might want it to be. While other tools may compare funds only to the S&P 500 (or 500 Index fund), you can use this tool to determine how closely the performance of one Vanguard stock fund tracks that of any other Vanguard stock fund. Compare all the funds that you own. To diversify, look for funds that have low correlation with one another.