The article says Denmark: "For the study, researchers used data from a nationwide mental-health screening tool of children in Denmark – a survey that is widely

The article says Denmark: "For the study, researchers used data from a nationwide mental-health screening tool of children in Denmark – a survey that is widely used internationally in clinical and academic settings – and matched it against the Danish census. Linking the two allowed researchers to make analyses using exact dates of birth, allowing for robust comparisons among similarly aged children. Because children in Denmark enroll in kindergarten in the calendar year they turn six, kids born exactly on Dec. 31 would have started kindergarten earlier that year, while those born a day later on Jan. 1 would be 6-years-and-8-months old when they start formal schooling. Researchers were thus able to examine differences between children who were born a certain number of days before the Dec. 31 enrollment cut-off and those born after the cut-off. They used full-sample sizes from the parent-reported mental health survey; 54,241 parents responded when their children were age 7 and 35,902 responded when the children were about 11 years old."