The Childcare Database: a valuable register linkage.

RATIONALE: To assess the impact of childcare attendance on morbidity for an entire country the Childcare Database was created by linking person-identifiable data from several Danish national data sources. The present paper describes the creation, characteristics, and potential of the Childcare Database. DESIGN AND MEASUREMENT: Based on the unique Danish Civil Registration System (DCRS) identification number register data were linked regarding childcare attendance, child and family characteristics, childcare facility characteristics, and hospitalizations for children aged 0 to 5 years in the period from 1989 to 2004. POPULATION AND SAMPLE SIZE: The number of children in the database increased in the period from 1989 to 1998 as more municipalities started registering childcare, whereafter it stabilized. The maximum number of children in the database was 373,142 in 2000. From 1999 to 2004 the Childcare Database covered more than 90% of all 0- to 5-year-old children in Denmark. At age 1 year 53% of children were enrolled in a childcare facility and approximately 75% were enrolled from 3 to 5 years of age. POTENTIAL: The Childcare Database offers a unique potential for epidemiological research within the area of childcare and morbidity. Future updates and linkages with other data sources will further expand the potential of the database. Due to the high quality of the linked nationwide register data, the Childcare Database enables conduction of epidemiological studies of high internal and external validity.