Asset pre-selection for a cardinality constrained index tracking portfolio with optional enhancement

An index tracker is a passive investment reproducing the return and risk of a market index, an enhanced index tracker offers a return greater than the index. We consider the selection of a portfolio of given cardinality to track an index, both without and with enhancement. We divide the problem into two steps - (1) pre-selection of assets; (2) estimation of weights on the assets chosen. The eight pre-selection procedures considered use: forward selection (FS) or backward elimination (BE); implemented using ordinary least squares (OLS) or least absolute deviation (LAD) regression; with a regression constant (c) or without (n). The two-step approach avoids the NP-hard problem arising when asset selection and asset weight computation are combined, leading to the selection of a cardinality constrained index tracking portfolio by computer intensive heuristic procedures with many examples in the literature solving for portfolios of 10 or fewer assets. Avoiding these restrictions, we show that out-of-sample tracking errors are roughly proportional to 1/sqrt(cardinality). We find OLS more effective than LAD; BE marginally more effective than FS; (n) marginally more effective than (c). For index tracking, both without and with enhancement, we use BE-OLS(n) in sensitivity analyses on the periods used for selection and evaluation. For a S&P 500 index tracker, we find that out-of-sample tracking error, transaction volume and return-risk ratios all improve as cardinality increases. By contrast for enhanced returns, cardinalities of the order 10 to 20 are most effective. The S&P 500 data used from 3/1/2005 to 29/12/2023 is available to researchers.