knife river's backlog is comprised of the anticipated revenues from the uncompleted portion of services to be performed under job--specific contracts. a project is included in backlog when a contract is awarded and agreement on contract terms has been reached. however, backlog does not contain contracts for time and material projects that a fixed amount cannot be determined. backlog is comprised of: (a) original contract amounts, (b) change orders for which customers have approved and (c) claim amounts that have been made against customers for which are determined to have a legal basis under existing contractual arrangements and as to which recovery is considered to be probable. such claim amounts were immaterial for all periods presented. backlog may be subject to delay, default or cancellation at the election of the customers. historically, cancellations have not had a materially adverse effect on backlog. due to the nature of its contractual arrangements, in many instances knife river's customers are not committed to the specific volumes of services to be purchased under a contract, but rather knife river is committed to perform these services if and to the extent requested by the customer. therefore, there can be no assurance as to the customers' requirements during a particular period or that such estimates at any point in time are predictive of future revenues.