Think of the Social Security Administration and you may envision a high-tech automaton that magically disburses millions of retiree checks every month.

But when it comes to adjusting individual Supplemental Security Income payments, agency employees often do "manual computations" with uneven results, according to a new report by the agency's inspector general.

Drawing on a random sample of recipients who collected payments between July 2006 and June 2008 tied to at least one manual computation, the IG concluded the calculations were wrong in about 30 percent of those cases. As a result, some 14,400 recipients were overpaid a total of $7.7 million while almost 18,500 were underpaid about $6.7 million, the report estimates. Some recipients could have been overpaid at one point and short-changed at another.

The manual calculations generally are necessary when there are changes in an SSI recipient's eligibility status, said Jonathan Lasher, spokesman for the IG. "If their living arrangement or other factors change and retroactive adjustments need to be made . . . the computer system that administers that is not set up for that."

SSI channels benefits to some 7.8 million financially needy people who are "aged, blind or disabled," according to the most recent statistics.

"The inaccurate manual computations most likely occurred because an SSA employee did not properly consider all factors that affected the resulting SSI payment amount, as required by SSA policy," the report says. Although a second Social Security employee is supposed to check the numbers, that wasn't always done, the IG found. Even when it was, the result still sometimes turned out wrong.

In a written response attached to the report, the agency agreed to correct payment errors for recipients in the IG's sample and provide refresher training for employees.

The factors affecting SSI payments included living arrangements, income and other resources. One example outlined in the report suggests why it might be difficult to write a computer program encompassing them all.

In that case, an SSI recipient separated from her husband, who was also on SSI as well as disability insurance. After they split up, a Social Security employee — using a manual computation — correctly reckoned the effect on her benefits of the loss of the husband's disability insurance income and the fact that she was no longer in a household where both members receive SSI.

The error came because the husband kept paying the rent even after the separation. The Social Security Administration counted that as unearned income and thus reduced the wife's SSI payment. But it did so two months prematurely, costing her a total of $416.