Each dataset provides monthly data at the national level for initial Social Security Disability Insurance (SSDI) applications filed via the Internet. The dataset includes only SSDI initial receipts. SSDI initial receipt represent a worker in covered employment long enough and recently enough to be "insured", earning paid Social Security taxes. Social Security work credits are based on total yearly wages or self-employment income. The amount needed for a credit change from year to year. The number of work credits needed to qualify for disability benefits depends on age at disability onset. Generally, 40 credits are needed, 20 of which were earned in the last 10 years ending with the year of disability onset.