The major problems with using secondary data sources are that the data may not be uniform across the entire study population. For example, birth certificates and death certificates may not be available for everyone. Sometimes if the request for the data is old enough, the information has been deleted or lost. Furthermore, the data source may not provide uniform information (not every death certificate or birth certificate is the same). Much of the information listed on these data sources can be alterred. For example, in order to obtain a birth or death certificate certain information needs to be provided by the family. If the source is not so forthcoming, then the data can be false or misleading.
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