Description. Tracking Error is a measure of how closely an ETF follows its benchmark index. This data point seeks to isolate the ability of the portfolio managers to track the index, ignoring the confounding effects of ETF liquidity on market prices.escription. Tracking Error is a measure of how closely an ETF follows its benchmark index. This data point seeks to isolate the ability of the portfolio managers to track the index, ignoring the confounding effects of ETF liquidity on market prices.