The story about computer checkout systems in last week's Food section contained a mathematical error. On the average a Giant store which installs the computer system will save the company $6,000 per month, not per week.

"I'm not opposed to modernization," the voice on the other end of the phone concluded, after cataloging his experiences with one of the computer checkouts at a local supermarket: Cans of cat food marked 6 for $1.85 were rung up as $1.89; cookies on sale for 53 cents were recorded at their non-sale price of 63 cents; loaves of bread marked 2/79 at the shelf cost 49 cents each according to the machine.

"Fortunately," continued the outraged shopper, "they (the supermarket chain) have not been able to take the prices off. If they had, I wouldn't have known. The potential for unscrupulous practices is large. This is the real justification for keeping prices on each products."

The man had obviously come a cropper of a faultily programmed commputer. Even if "computers don't make mistakes" (which really isn't true: They do malfunction), the people who program them certainly can and do.

Shoppers who patronize Giant supermarkets have become increasingly aware of computer-assisted checkout in the last year. The Washington-based chain was one of the first in the nation to experiment with it. The first computerized Giant opened four years ago this month in Severna Park, Md.

A scanner, connected to the electronic cash register, "reads" the Universal Product Code (UPC), a series of lines and numbers found on almost every item sold in a supermarket. The numbers on the UPC tell the computer the brand name of the product, the particular item and the price. The scanner doesn't have to see the price marked on the item as a human checker would. The cash register flashes the price on a screen and produces a register tape with far more detailed information than in the past. For example, two packages of Jell-O, which sell for 55 cents, are printed out like this: JELLO RASBRY.28 followed by JELLO BLKBERY.27

Originally, the system's advantages for the shopper, such as the detailed record of purchases, as well as for the store, were overshadowed by an acrimnious struggle over whether or not each item on the shelves shold continue to be individually marked.

Consumer protests resulted in many local jurisdictions, including several states, passing laws making individual price marking mandatory. Federal legislation was pending when the supermarket industry gave in.

A 1976 study of shopping habits funded by the supermarket industry confirmed what the consumer groups had been trying to tell them: People wanted the prices to tell them: People wanted the prices on and they lost price consciousness when individual items were not market.The $75,000 Michigan State survey showed that a significant number of shoppers -- over 15 percent -- switched from stores without individual price marking to stores which used the traditional method.

Following the results of the study, a grocery industry committee recommended that members that have electronic equipment "Follow the same traditional aproach to individual item marking as is used in conventional supermarkets."

With most of the heat taken out of the debate, consumer groups turned to other mattes and the industry began in earnest to explore the advantages of computerized checkouts. But not too quickly.

The substantial capital investment per store for the system -- between $120,000 and $300,000 -- has kept most chains from adopting it. Only about 500 out of 33,000 supermarkets in this country have installed the equipment and Giant accounts for a significant number of them. By 1981, all of the chains' 115 markets will be computerized. Meanwhile, Safeway is scheduled to open its first local computerized checkout store this weekend in Fairfax City.

It offers significant advantages for the company, reducing labor costs, even though clerks must still price mark each item; tightening inventory control; increasing marketing efficiency and reducing "overrings" and "underrings" as they are called in the business. Consumers were promised faster checkout, which seems to have materialized in the Giant stores.

Without doubt the opportunity for the cashier to make mistakes -- possibly even deliberate on occasion -- is reduced. Of complaints from people who have suddenly become aware of their detailed register tapes and were comparing the prices marked on the items they had been charged, one of the frequent problems involves the "twofers" and "threefers," items. For example, various flavors of yogurt that are 2/79 cents, or 3/ $1.

If the computer is not programmed to "mix and match" the flavors, two flavors of the 2/79 yogurt can, and do, (see illustration on Page E1) cost 40 cents each, while three different flavors of the 3/ $1 yogurt have cost shoppers 34 cents each, for a total of $1.02, or 33, 34 and 34 cents respectively, which comes out to $1.01.

Problems also can arise when the scanner is unable to "read" the UPC. In one instance one can of peas and carrots (2/79 cents) passed over the scanner at 40 cents. The second can would not "scan." Giant says its cashiers are instructed to pass an item over the scanner twice. If the price does not register, the cashier must ring it up manually.

Ringing up the second can of peas manually, the checker charged 40 cents for it, bringing the total cost of the 2/79 item to 80 cents.

These are instances involving pennies, to be sure; but they and up, not only for the customer, but for the store as well.

Because the prices flash on the screen so quickly, it is almost impossible to compute the total price for these items in your head, even if you have them next to each other on the conveyor. This means you must double check when you get the groceries and the register tape home. That's an additional chore not everyone is eager to spend time doing.

On another occasion, six cans of frozen grapeftuit juice, were purchased. At the checkout counter the shopper discovered that one can was 59 cents, the other five, 61 cents. This discrepancy was called to the cashier's attention who said she would have to charge whatever was in the computer. That turned out to be 61 cents. A trip back to the frozen food case uncovered five additional 59 cents cans of frozen grapefruit juice. A prolonged argument between the customer and the man in charge of the store ensued. Eventually, he gave the customer the difference -- 12 cents.

When this what brought to the attention of Giant officials, this was their response:

"That is ridiculous, asinine. Our policy," said Gerson Barnett, a Giant vice president, "is to give that customer what she brought to the front end at the lower price. Then you go back to the office and find out what the problem is and correct it."

Reminded that if Giant had stopped price marking individual items, only someone with a photographic memory would have known about those discrepancies, Barnett agreed. Reminded that consumer groups had to wage an all-out campaign to make sure price marking continued, Barnett refused to acknowledge there had been a battle, one which has left some scars. Said Barnett: "We were willing to do what the consumer wanted. If she wants the prices on we'll have the prices on. There's no question about that."

Even with individual price marking, Giant appears to be recouping its investment quickly. Barnett said the capital outlay, about $250,000 per store would be paid for in "three or four years. We're talking about average savings of $6,000 per store per week."

Commented an official of the supermarket industry: "Any kind of businessman would go for that kind of investment if he had the capital to start with."