In essence, the eligibility criteria for both SSDI and SSI is as follows: Are your symptoms so bad that they impair your normal daily living activities to such a degree that you cannot perform what Social Security calls "substantial gainful employment" - any employment for which your age (25 years under the age of 65) education, and past work experience qualify you to perform, for which you can earn - in 2009 - a minimum of $980/month. For SSI there are additional financial criteria.
In otherwords, SGA - Substantial Gainful Employment - is any occupation for which you can be paid about $12.25/hour, 20 hours/week. If you can do such work and earn such money, you are ineligible for either SSDI or SSI.
For a FREE ebooklet that explains this concept in great detail, please email me at this link to my website: www.disabilitykey.com/contact.htm