In late January 2005, the allocation (pro-rata) algorithm was enhanced to introduce an element of  FIFO matching to the pro-rata algorithm. This hybrid matching algorithm provides greater opportunity for traders who enter the market early at a given price, while significantly increasing efficiency by reducing the number of fill messages. Details of the algorithm are as follows:

• Fractional Fills. If the allocation results in a fractional fill, the CME Globex platform will continue to round down the fractional contract and it will be combined with all other fractional fills for allocation during a second "pass" through the order book.
  • The remaining contracts resulting from fractional fills are allocated according to FIFO rules.
• Allocation of 1 Lot Fills. Any orders that would result in a 1 lot fill are not filled during the first pass through, and these 1 lot fills are added to the fractional fills to be allocated FIFO.

Please note: TOP order priority and the pro-rata allocation of trades after filling the TOP order has not changed. Any order that improves the best bid or offer in the book will be filled in its entirety before other orders are filled.

Pro-Rata Algorithm for CME Eurodollar Futures Example

• Assume Orders 1 through 5 were entered in the following time sequence according to order number and represent the best bid in the order book
• Assume an incoming offer to sell 60 lots at 97.04 is entered
• The following chart illustrates the CME Globex allocation:

Incoming order to sell 60 at 97.04

1. Order 1 is allocated 10 lots based on its TOP order status.
2. Orders 2 - 5 are allocated fills according to the percentage they comprise of the remaining 150 contracts on the bid, multiplied by the 50 lots remaining of the incoming offer. All fractional fills are rounded down. As you can see in the gray shaded column on the right, Order 2 would have resulted in a 1 lot fill (1.666 would be rounded down to 1 lot) and as a result is allocated zero contracts on the first pass according to the new matching rules.
3. The first pass through the order book results in the allocations illustrated above with 3 lots left to be allocated due to rounding and the omission of the 1 lot fill.
4. The remaining 3 lots will be allocated according to FIFO rules which will result in Order 2 receiving the 3 lot fill.

As you can see, the new matching rules are beneficial to the relatively smaller Order 2, which was quicker to the price than Orders 3-5.

Please see the CME Globex Price Banding by Product for the list of products which use the allocation (pro rata) matching algorithm.

Return to Matching Algorithms.