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In this letter, we present exact statistical expressions for opportunistic amplify-and-forward (AF) relaying under a tight power constraint, where a best relaying node is selected based on partial channel information (PCI) or full channel information (FCI), respectively. Based on the statistical analysis, we compare PCI with FCI strategy in terms of outage and average capacity. Numerical investigation shows that, as the number of candidate relays increases, the performances with FCI are continuously improved while those improvements with PCI are bounded. However, for the small number of the relays at a low SNR region, PCI and FCI achieve very similar performances. The numerical results also show that the relaxed power constraint that ignores the noise power at the relay leads to significant deviations from the tight constraint, which is the more reasonable one, especially in computing the outage with FCI.