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In page Professional Coin Grading Service:

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Analysis of the population report, and a similar report published by NGC, has allowed rarity estimates to be made of specific coins. Over time these two data bases have revealed some coins once thought rare to be remarkably common, whereas others thought more common have shown to be likely few in number. The population reports are followed closely by numismatic professionals, who recognize that population numbers can be inflated through multiple submissions of the same coins broken out of their holders and resubmitted with the hope of receiving a higher grade. Population figures can also be artificially low due to the reluctance to submit inexpensive coins—for a service that may cost more than the coins are worth.[3][4]