The Agriculture Department's allotment of food stamps for a typical U.S. family with no other income is not enough to cover the cost of USDA's minimum monthly food plan in major American cities, an advocacy group charged yesterday.

Michael Lemov, director of the Food Research and Action Council, said a FRAC survey of 309 grocery stores in the District and seven other major cities found that it would cost a family of four an average of $348.48 to buy the USDA's "Thrifty" food plan, purchasing national brands off the shelf. But, it noted, a family of four with no other income receives only $268 worth of food stamps.

The food plan ranged in cost from a low of $377.20 a month in Philadelphia to a high of $412.88 in the District.

Lemov said the department, using its national surveys, has estimated that the plan would cost only $268 a month, which it uses as the basis for its food stamp allotment.

Lemov said the FRAC survey, supported by a grant from the American Can Co. Foundation, shows that "poor people are being shortchanged on food stamps. This helps explain why people are running out of food stamps by the end of the month and are going to food kitchens and pantries to supplement their food." The FRAC survey, taken during the summer, was based on food purchases for a week's portion of the Thrifty budget in each city.

Betty B. Peterkin of USDA's Human Nutrition Information Service, said the department could not explain the cost discrepancies without a detailed analysis of the FRAC survey.

But she said a preliminary look suggested that FRAC had overestimated by tenfold the amount of ground beef called for in the Thrifty budget, adding $11 a week to costs, used higher-priced breakfast cereals instead of more moderately priced ones and had overestimated needed supplies of certain oils and mayonnaise.

However, FRAC disputed these contentions, saying that, for example, it had substituted the higher level of ground beef for other meat, poultry or fish components to the Thrifty plan.

According to the FRAC survey, the cost of the Thrifty plan, which the department estimates is the minimum adequate food a family needs for a month, was $381.04 in Boston, $383.28 in Chicago, $377.56 in Detroit, $399.76 in Houston, $384.12 in New York and $400.60 in San Francisco, when national brands were purchased. If store-name brands were used, the monthly cost was $20 to $60 a month less, with Detroit the lowest at $338.36 -- but still far higher than the $268 food stamp allotment.

Under the food stamp program, low-income families are allowed to deduct from their income up to 18 percent of earned income, plus $98 a month as a standard deduction, plus certain amounts for extra-high rents and child care.

If that amount comes to zero, they are allowed to obtain food stamps equal to the full cost of Thrifty budget.

However, if their income after deductions is more than zero, they are expected to spend 30 percent of it on food, and they get food stamps to make up the difference between that amount and $268 -- the cost of the Thrifty plan.

Thus, a family of four with gross income of $446 a month -- about average for recipients -- might have net income of $260 after deductions.

In that case, it would be expected to spend $78 out of pocket for food (30 percent of $260) and it would get $189 in stamps, bringing its food budget up to about $268 (figures are rounded).

Lemov said that it is absurd to think that a family with $446 gross income a month, receiving $189 in food stamps, could pay another $200 or so a month out of pocket to cover the average of $384.48 that FRAC said is needed to buy the Thrifty food plan.