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Wireless sensor networks place sensors into an area to collect data and send them back to a base station. Data fusion, in which collected data are fused before they are sent to the base station, is usually implemented over the network. Since a sensor is typically placed in locations that are accessible to malicious attackers, information assurance of the data fusion process is very important. A witness-based approach by Du, W, et al (2003) has been proposed to verify the fusion data. In this approach, the base station receives the fusion data and "votes" on the data from a randomly chosen sensor node. The vote comes from other sensor nodes, called "witnesses," to confirm the correctness of the fusion data. Since the base station receives the vote through the chosen node, this node could forge the vote if it is compromised. Accordingly, the witness node must apply cryptographic operations to the vote to prevent this forgery. The cryptographic operation requires more bits than the vote, increasing the transmission burden from the chosen node to the base station. The chosen node consumes too much power. This work improves the witness-based approach using a direct voting mechanism such that the proposed scheme performs better in terms of assurance, overhead, and delay. The witness node transmits the vote directly to the base station. Forgery does not pose a problem in this scheme. Moreover, fewer bits are necessary to represent the vote, significantly reducing the power consumption. Performance analysis and simulation results indicate that the proposed approach has a 40-times lower overhead than the witness-based approach.