R Squared, Definition
R squared is used to measure the extent to which an investment, such as a mutual fund, follows a given index. A R squared measure of 0.85 means that 85% of the movement in the fund is attributable to the index which it most closely follows.
An index fund or ETF should have an R squared of 1.00 or very close to that. If an actively managed fund has an R squared of higher than 0.85, that fund falls into the dubious category of Closet Index Fund