The acronym SSI stands for Supplemental Security Insurance. Under the system, an individual receives a set amount of money per month. If there is an SSI decrease in the monthly sum, this may be the result of increases in other income sources. If an individual receiving SSI obtains gainful employment, the amount of money earned may result in a reduction of the amount of the SSI check. This type of SSI decrease is based on a formula. It allows the individual to earn up to a specific amount of money for a certain period before the SSI decrease comes into effect. Another reason for an SSI decrease is a change in status or classification. If an individual is classified as a full-time disables SSI recipient, the amount will change if he or she is reclassified as part-time disabled recipient. Talk to a lawyer specializing in supplemental and social security issues for clarification of your rights.