Social Security uses a sequential evaluation process that essentially acts like a flow chart for SSDI and adult SSI cases; meaning if you pass one step, you move on to the next evaluation. The steps are broken down below:
Step 1 - Work
Is the claimant earning enough money working to be considered "substantial gainful activity"? If so, claim is denied. If not, move on to step 2.
Step 2 - Severity
Is the claimant's disability severe enough and meet durational requirements? If not, claim is denied. If so, move on to step 3.
Step 3 - Medical Listings
Does the claimant's disability meet or equal one or more of Social Security's medical listings? If so, claim is approved. If not, move on to step 4.
Step 4 - Previous Work
Can a person with the claimant's disabilities perform the claimant's past work? If so, claim is denied. If not, move on to step 5.
Step 5 - Any Work
Considering the claimant's age, education, and work history, are there any jobs in the national economy the claimant could work? If so, claim is denied. If not, claim is approved
This is obviously a very simplified version of Social Security's evaluation process, but hopefully it is helpful for you in understanding how they approach a claim. For a more complete version, try visiting ssa.gov or check out this report from the United States Government Accountability Office.