The Agriculture Department yesterday angrily denied an advocacy group's assertion that it is shortchanging food stamp recipients by underestimating the cost of a minimum adequate diet for a family of four.

John W. Bode, assistant secretary of Agriculture for food and nutrition services, said a study by the Food Research and Action Center "has so many major errors that its conclusions must be dismissed entirely." The study had said that it cost an average of $384.48 a month to buy the department's so-called Thrifty Food Plan, rather than the $268 that USDA estimated.

The errors, according to Deputy Assistant Secretary Suzanne Harris, stem from the fact that in its survey of 309 stores in eight cities, the group bought much larger quantities of food than the department had specified in the plan, which is considered the minimum necessary to meet a family's nutrition needs.

When it released its survey last week, FRAC said the actual cost of the Thrifty plan for four people is $384.48 a month if off-the-shelf national brands are purchased, and about $40 less if store brands are used. Because a family of four that is classified as having no cash income receives $268 a month in food stamps, FRAC said it would not be able to afford an adequate diet.

The department, in a detailed analysis of the foods on FRAC's purchase list, which it said appeared to be based on a sample one-week menu the department published two years ago, said the group had bought far too much of some items.

The department said FRAC's list appeared to be based on a sample one-week Thrifty plan menu the department published two years ago, which called for about 10 pounds of meat, fish and poultry. FRAC's one-week purchases, however, totaled 16.5 pounds.

The department said its one-week sample menu called for 24 pounds of fruits and vegetables, but FRAC purchased 33 pounds. It said its sample menu called for 29 ounces of fats and oils, but FRAC purchased 72 ounces.

The department said that using FRAC's prices but the amounts of food in its sample menu, it figured it would cost a family of four $66.66 a week, using national brands, or about $288 a month. Using store-brand prices with the proper quantities, the same menu would cost $57.64 a week or about $249 a month. A mix of national and store brands, it said, would produce costs of $268.

Kathleen McKee, attorney and research coordinator for FRAC, defended the FRAC survey, saying the group had purchased a few items in somewhat larger quantities than called for in the Thrifty plan. But in most cases, she said, the group had simply substituted larger quantities of one food for an alternative food that was eliminated.