An employee, widow(er), or child is disabled for any regular employment if he or she is unable to do any substantial gainful activity because of a medically determinable physical or mental impairment which meets the duration requirement defined in § 220.28. In the case of a widow(er), the permanent physical or mental impairment must have prevented work in any regular employment before the end of a specific period (see § 220.30). In the case of a child, the permanent physical or mental impairment must have prevented work in any regular employment since before age 22. To meet this definition of disability, a claimant must have a severe impairment, which makes him or her unable to do any previous work or other substantial gainful activity which exists in the national economy. To determine whether a claimant is able to do any other work, the Board considers a claimant's residual functional capacity, age, education and work experience. See § 220.100 for the process by which the Board evaluates disability for any regular employment. This process applies to employees, widow(er)s, or children who apply for annuities based on disability for any regular employment. This process does not apply to surviving divorced spouses or remarried widow(er)s who apply for annuities based on disability.