Here are some key factors you should consider while investing in ETFs:\r\rExpense ratio: When choosing between two ETFs with the same benchmark, say Nifty 50, you should give preference to the ETF with a lower expense ratio. The general rule is, the lower the expense ratio, the better.\rTracking error: The returns given by an ETF may not exactly match the returns given by the benchmark. It happens because the fund manager may keep some cash to meet daily operations. The difference between the benchmark returns and the ETF returns is known as tracking error. When choosing between two ETFs with the same benchmark, say Nifty 50, you should give preference to the ETF with a lower tracking error. The general rule is, the lower the tracking error, the better.\rAssets Under Management (AUM): You should ideally choose an ETF with a higher AUM. Bigger schemes may be subject to lower volatility. However, you should always give more preference to expense ratio and tracking error than AUM while choosing ETFs for investment.