Survey error (variance and bias) that is introduced when there is not a one-to-one correspondence between frame and target population units. Causes include when some units in the target population are not included on the sampling frame (undercoverage), when some units on the sampling frame are not members of the target population (out-of-scope), when more than one unit on the sampling frame corresponds to the same target population unit (overcoverage), and when one sampling frame unit corresponds to more than one target population unit.

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