Investors who are happy with the benchmark's index returns minus the expense ratio of the ETF tend to stick with market-cap weighted ETFs and attempt to add alpha through overweighting growth or value around the benchmark, according to their market prognostications. How do market-cap weighted ETFs such as the S&P 500 Index SPDR ( SPY), iShares S&P 500 Index ( IVV), iShares Russell 2000 ( IWM) and Vanguard Large-Cap ETF ( VV) rebalance? Well, the short answer is they don't rebalance, as there is no pre-defined rebalancing schedule within the market-cap methodology. Unfortunately, if one's motivation is to outperform his or her benchmark, using market-cap-weighted ETFs requires precise timing on when to be long the market and when to be in cash because simply buying and holding a market-cap-weighted ETF will lose to the benchmark 100% of the time due to the expense ratios associated with holding the ETFs. In essence, market-cap-weighted ETFs are rebalanced by the market's fluctuations in price, and as one university endowment manager once aptly stated to me, "they rebalance by attrition" which is certainly food for thought. Rydex competes with the market-cap-weighted ETFs with S&P 500 Equal Weight ETF ( RSP). The underlying constituents are the exact same 500 names that are in SPY and IVV, but this ETF actively rebalances the portfolio holdings, as the market pushes them out of line. Unlike SPY or IVV, in which market forces drive the individual weightings of the securities up or down within the ETFs, RSP will rebalance the underlying index on a quarterly basis to keep all 500 members with equal weightings. In between the quarterly rebalances, of course, RSP functions just like a free-float market-cap index, and certain names or sectors may surge while others fall, thus changing the weightings from top to bottom within the ETF between rebalance periods. Why rebalance quarterly and why equal-weight your index? If you examine the five-year trailing returns of SPY, IVV and RSP, you will see RSP truly has added alpha against the benchmark, up 2.49%, with SPY down 8.80% and IVV down 8.65% during this period. The five-year performance can be seen here. So it seems that a regular rebalance like the RSP employs adds value, but it certainly doesn't come without cost. RSP charges 40 basis points, while SPY and IVV charge 9 basis points. I urge portfolio managers not to judge the quality of an ETF by cost alone. Instead, assess whether the ETF is designed to and has demonstrated the ability to outperform its stated benchmark. If it has, then simply factor in the expense ratio to net out this outperformance, and if you are still ahead of your benchmark, then the ETF is certainly worth further examination and consideration for your portfolios.