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In peer-to-peer (P2P) overlay networks, a group of multiple peers have to cooperate with each other. P2P systems are in nature scalable distributed systems, where there is no centralized coordinator. It is difficult for each peer to communicate with every other peer. An acquaintance peer of a peer is another peer with which the peer can directly communicate. Each peer has to obtain access and location information on resources through communicating with acquaintances. It is critical to discuss how each peer can trust an acquaintance since acquaintances may have obsolete information. There are subjective (direct) and objective (indirect) types of trustworthiness of a peer on an acquaintance. A peer obtains the subjective trustworthiness on an acquaintance through directly communicating with the acquaintance. Here, the more number of satisfiable replies a source peer receives from a acquaintance, the larger subjective trustworthiness on the acquaintance the source peer has. On the other hand, a peer obtains the objective trustworthiness on a target acquaintance through collecting subjective trustworthiness on the target acquaintance from other peers. Here, the subjective and objective types of trustworthiness on an acquaintance might be different. That is, other peers have different trustworthiness opinions on the target acquaintance. A peer decides on which type of trustworthiness to be taken based on the confidence. The confidence of a peer shows how much the peer is confident of its own trustworthiness opinion, i.e. subjective trustworthiness on the acquaintance. If a peer is confident of the trustworthiness opinion, i.e. the confidence is larger, the peer takes the subjective trustworthiness on the target acquaintance. Otherwise, the peer takes the objective trustworthiness.