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In page One-name study:

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A full one-name study can be daunting, particularly if the surname is very common. Conversely, a rare surname can be difficult to trace.[1] Since such studies are usually conducted by individuals as a pastime, they are generally feasible only when a surname is not used by more than a couple of thousand contemporary people, so that the total historical data-set is numbered in the low tens of thousands. Where a surname is used by hundreds of thousands, or millions of people, it would be practically impossible to differentiate these persons using national-index data alone.