Advertising claims are, alas, sometimes greeted with skepticism. However, one new ad campaign appears to me to be not only believable but quite accurate in its claims.

I refer to the recent ads that have been telling us that we can save money by switching to "instant" coffees. Instinct tells us that instant coffee is a convenience food; something additional has been done to it, and therefore it ought to cost more. Right?

Wrong. The ads say it costs less to use instant, and I say, too.

What makes me a maven on coffee prices? Nothing, really. But it happens that I became interested in the subject long before the instant coffee people began making their economy claims, and for two months I have been engrossed in one of my highly unscientific experiments. Let me tell you about it.

I was in a supermarket helping my first wife with her marketing one day when she said, "Go over to aisle 10 and get a pound of your decaffeinated coffee." Little did I know what a shock was in store for me there.

"Good heavens," I said to good old Whatshername when I brought her the coffee. "The price of coffee has gone out of sight since the last time I was in a supermarket."

"Yes, dear," she said. "Maybe you ought to go marketing more than once a year."

While she began looking at meats, I went back to the coffee display and began checking prices - one brand against another, 1-pound cans vs. 2-pound cans, regular coffee against decaffeinated, and finally ordinary ground coffee against "instant." And there my comparison shopping came to a grinding halt. How the heck can one compare teaspoonfuls to pounds?

I suppose some whiz kid with a degree in third-grade arithmetic could figure it out with a pencil and paper, but not I. When you ask me, "If you add two apples to three apples, how many apples would you have?" I go to the refrigerator, get out a bag of apples, and figure it out the old-fashioned way. And my coffee experiment was conducted in the same way.

I usually make the coffee at our house because I can get used to almost any kind of coffee if you keep serving me the same thing every day, but my wife scoffs at this notion. "A good cook doesn't measure," she says haughtily. So I make the coffee - and I measure. Each day I make 20 ounces of coffee for each of us.

I used ground coffee beans to make regular for her, decaffeinated for me. Either way, I found that three scoops of coffee to 20 ounces of water gave us the strength we like, and that there are about 32 scoops of coffee in a 1-pound can. So a pound lasts for just about 11 days.

On Jan. 12, I opened a four-ounce jar of instant decaffeinated coffee and began using that in brewing my 20 ounces. It didn't take long to learn that two teaspoonfuls - each heaped up to 80 or 85 per cent of absolute capacity - produced an instant decaffeinated - produced and instant decaffeinated coffee of "adequate" strength and flavor. Once they tell you that you can't drink real coffee any more, nothing you substitute will be precisely the same, but some substitutes can qualify as adequate, and this concoction is one of them.

The bottom line to all of this is that my four-ounce jar of instant coffee lasted for 28 days and finally gave out yesterday. I produced as much drinkable coffee as 2 1/2 pounds of ground coffee beans - but it sure didn't cost as much as 2 1/2 pounds of ground coffee beans. It was, just as the advertisements claim, a good bargain.

If you are also struggling to keep your food budget at a reasonable level, you might want to try one or more of the instants. If you do, experiment a bit. Instead of pouring boiling water into a cup that contains a teaspoon of instant coffee powder, I put the coffee powder into the boiling water and stir as I let it simmer for a bit. I tell myself that I get better flavor that way, but maybe not. Do your own experimenting, and you may be in for a pleasant surprise.