When comparing index funds, it is important to consider several factors. These include the fund's expense ratio, which reflects the annual operating costs as a percentage of the fund's assets. Lower expense ratios are generally preferred, as they have a direct impact on investment returns. It is also essential to review the fund's tracking error, which measures how closely the fund's performance aligns with the index it aims to replicate. Historical performance and diversification across different market segments are additional factors to consider.