The definition of the prevalence of ADHD diagnoses is based on information from THL's Hilmo registers. The statistics take into account children (7–12-year-olds) and adolescents (13–17-year-olds) who have visited primary health care (AvoHilmo, incl. school health care) or specialized medical care (TerveysHilmo). A case is a person for whom an ADHD diagnosis is found as the reason for the visit or a principal, an additional or a long-term diagnosis in connection with the treatment record at least once during a calendar year. The criterion for defining cases was any diagnostic code referring to ADHD (F90 or F98.8) in the ICD-10 classification of diseases in the Register of Primary Health Care or Specialized Medical Care. The same person was only taken into account once in the national statistics if they had multiple visits during the calendar year, in which the reason for the visit was the ADHD diagnosis based on the information from the Care Registers. If the person had moved to another wellbeing services county during the calendar year, they were taken into account separately for each county when calculating the number and prevalence of cases (in proportion to the population, %).
The population figures are based on the official population figures from Statistics Finland. The population living in Åland is included in the figures for the entire country. The population estimate used is the population situation in the last week of the calendar year.