Selection bias encountered in the systematic linking of historical census records

Linked historical records typically are unrepresentative of the population from which they are drawn even if the method of linking is restricted to time-invariant matching criteria. An example drawn from Canadian census records illustrates the nature of bias that may afflict even a carefully linked sample. The use of potentially time-varying match criteria doubles the size of a linked sample at a modest cost in terms of additional bias. This trade-off is attractive for some research purposes if care is taken in the uses to which the data are put. Reweighting to mitigate the effects of bias in visible characteristics is desirable.