http://social-security.lawyers.com/s...isability.html

"Disability is determined using a five-step sequential evaluation process conducted by the SSA. If, at any point, an applicant is found not to be disabled, the evaluation process terminates and the claim for disability insurance benefits is denied. Following are the five requirments in determining disability:

•Substantial gainful activity. If the applicant is currently engaged in substantial gainful activity, there is no disability, regardless of medical condition, age, or work experience.
•Severe impairment. If the applicant is not engaged in substantial gainful activity, the SSA determines whether the applicant has a severe impairment. An impairment is considered "severe" if it significantly limits a person's physical or mental ability to do basic work activities. If there is a finding of severity, the evaluation proceeds to the third step.
•Listing of impairments. If the applicant's condition meets the requirements, or is the equivalent of a disability on the SSA's Listing of Impairments, then the applicant is ruled disabled. If the applicant does not meet the requirements, the sequential evaluation process continues to the fourth step.
•Past relevant work. A medical assessment is performed to determine whether the impairment prevents the applicant from performing his past relevant work. If the applicant is found to be able to perform past relevant work, the claim will be denied. If not, the evaluation process continues to the final step.
•Other work. The SSA evaluates whether the applicant can perform other available work existing in significant numbers in the national economy. The evaluation considers the applicant's residual functional capacity (what the applicant is able to do in a work setting despite the impairment), age, education, and past work experience. If an applicant cannot perform other work, they will be found disabled."