Explain why an index portfolio can drift from its benchmark and how that tracking error is kept in check

Tracking error is the standard deviation of the difference between portfolio and index returns; the difference itself is the excess return. Sources include fees and commissions, cash drag from dividends and inflows not yet invested, index reconstitutions and corporate actions that force trades, sampling or optimisation that falls short of full replication, and market impact when trading illiquid names. Controls include equitising cash with index futures or ETFs, trading at the rebalancing close, anticipating reconstitution, replicating fully where cost allows and keeping turnover low; securities lending income can offset costs. The Level III point is the trade-off: pushing tracking error towards zero may cost more than the error it removes, so the manager balances closeness of fit against cost.