The rolling forward of the underlying futures contracts in the excess return index portfolio occurs once each month, on the fifth through ninth business days (the roll period). As explained above, some of the underlying commodity contracts expire in the next month and thus need to be rolled forward. The simplest way to think of the process is as rolling from one basket of nearby futures (the first nearby basket) to a basket of futures contracts that are further from expiration (the second nearby basket). The S&P GSCI™ is calculated as though these rolls occur at the end of each day during the roll period at the daily settlement prices.
The portfolio is shifted from the first to the second nearby baskets at a rate of 20% per day for the five days of the roll period. Until just before the end of the fifth business day, the entire S&P GSCI™ portfolio consists of the first nearby basket of commodity futures. At the end of the fifth business day, the portfolio is adjusted so that 20% of the contracts held are in the second nearby basket (i.e. a basket of future contracts that are farther from maturity), with 80% remaining the first nearby basket.
The roll process continues on the sixth, seventh and eighth business days, with relative weights of first to second nearby baskets of 60%/40%, 40%/60% and 20%/80%. At the end of the ninth business day, the last of the old first nearby basket is exchanged, completing the roll and leaving the entire portfolio in what we have been calling the second nearby basket. At this time, this former second nearby basket becomes the new first nearby basket, and a new second nearby basket is formed (with futures maturities further in the future) for use in the next month's roll.
The last key point to be made about the roll process is to specify exactly what the 80%/20% or other relative splits between nearby baskets mean. The roll percentages refer to contracts or quantities, not value. Taking the first day of the roll as an example, just before the roll takes place at the end of the day, the S&P GSCI™ consists of the first nearby basket. That portfolio, constructed the night before and held throughout the fifth business day, has a dollar value. For the roll, that dollar value is distributed across the first and second nearby baskets such that the number of contracts or the quantity of the first nearby basket is 80% of the total and the quantity held of the second nearby basket is 20% of the total.
The dollar value held of each nearby basket can then be calculated from those quantity weights by multiplying them by the prices of the futures contracts contained in each basket. As the baskets contain futures with different maturities for some of the commodities, the prices are generally close but not exactly the same. Hence, the percentage of the portfolio value (i.e. dollar weight) held in each basket is generally close to, but not exactly equal to, the 80%/20% split specified for the quantities.
The world-production weighting of the S&P GSCI™ is accomplished by keeping the quantity weights of the individual commodities within each basket proportional to world production weights, which are averages of historical production levels and are generally updated every year.