I get inquires regarding partial disability every now and then. Potential claimants believe they may qualify for partial Social Security disability benefits. They could still work a bit, but can no longer do the same job, or earn the same income, they earned before. Or perhaps, due to their medical condition, their earnings have dropped significantly because of lost time at work.
The Veterans Administration awards partial disability. And often in workmen's compensation claims, you will see a doctor's opinion that the patient has a certain percentage disability. But these are generally medical-based findings. A Social Security disability determination also has a vocational element.
With Social Security disability, it is all or nothing. Either you can work or you can't. If you are working and earning wages more than $1,000 gross pay per month (that's substantial gainful activity for 2010), then you do not qualify for disability (there are some limited exceptions, such as Impairment Related Work Expenses; so if your income is very close to $1,000 per month, do not assume that you can't qualify for disability; talk to a Social Security disability lawyer). There is no partial disability for Social Security.
When Social Security assesses your ability to work, they must assess your ability to work full-time, on a regular and continuing basis. So this "all or nothing" approach can benefit certain claimants, who may retain an ability to work part-time. Those claimants are still eligible for full disability benefits.