Revenue Recognition. Revenue recognition policies for specific sources of revenue are discussed below. Clearing and Transaction Fees. Clearing and transaction fees include per-contract charges for trade execution, clearing, trading on the company's electronic trading platform, portfolio reconciliation and compression services, risk mitigation, and other fees. Fees are charged at various rates based on the product traded, the method of trade, the exchange trading privileges of the customer making the trade and the type of contract. The majority of our clearing and transaction fees are recognized as revenue when a buy and sell order are matched. Therefore, unfilled or canceled buy and sell orders have no impact on revenue. On occasion, the customer's exchange trading privileges may not be properly entered by the clearing firm and incorrect fees are charged for the transactions. When this information is corrected within the time period allowed by the the company, a fee adjustment is provided to the clearing firm. A reserve is established for estimated fee adjustments to reflect corrections to customer exchange trading privileges. The reserve is based on the historical pattern of adjustments processed as well as specific adjustment requests. The company believes the allowances are adequate to cover estimated adjustments. Market Data and Information Services. Market data and information services represent revenue earned for the dissemination of market information. Revenues are accrued each month based on the number of devices reported by vendors or over a straight line basis in accordance with the market data subscription contract term. The company conducts periodic examinations of the number of devices reported and assesses additional fees as necessary. On occasion, customers will pay for services in a lump sum payment; however, revenue is recognized as services are provided. Other Revenues. Other revenues include access and communication fees, fees for collateral management and fees for trade order routing through agreements from various strategic relationships as well as other services to members and clearing firms. Revenue is recognized as services are provided. Concentration of Revenue. One firm represented 10% of the company's clearing and transaction fee revenue in 2018. One firm represented 13% and another firm represented 12% of clearing and transaction fees revenue in 2017. One firm represented 13% and another firm represented 11% of clearing and transaction fees revenue in 2016. Should a clearing firm withdraw from the company, management believes that the customer portion of that firm's trading activity would likely transfer to another clearing firm. Therefore, management does not believe that the company is exposed to significant risk from the ongoing loss of revenue received from a particular clearing firm. The two largest resellers of market data represented approximately 41% of market data and information services revenue in 2018 , 45% in 2017 , and 40% in 2016 . Should one of these vendors no longer subscribe to the company's market data, management believes that the majority of that firm's customers would likely subscribe to the market data through another reseller. Therefore, management does not believe that the company is exposed to significant risk from a loss of revenue received from any particular market data reseller.