EAVESDROPPING in a supermarket is wonderful. It can teach you more about peolpe's attitudes toward food and cooking than almost any book, and you can get your won shopping done while you're at it. We listened in recently as two women discussd the merits of one packaged chocolate cake mix over another, finally selecting the one whose TV commercial they remembered.

By the time the two had filled their baskets, they'd amassed a truly formidable collection of packaged foods: the cake mix, of course, plus frosting to smear from a can, pancake batter to pour from a carton, biscuit batter to swell from a tube and waffles to pop from a toaster, along with bottled salad dressing, canned soups and chili, spaghetti sauce in jars and enough frozen packages to cool a volcano. Provided their families didn't object (or know?), they were ready to serve up a week's worth of hot meals without cooking at all.

What is it that causes people to choose a convenience food over its homemade counterpart? There's the hard sell of the TV ads, of course, but potent as it is, it's just an influence, not a reason. So the question remains: Why? Part of the answer is surely the time that's saved in using a convenience food, or at least the perception that one is saving time. And since we've all been told that time is money, part of the rationale for the packaged product is that one pays a little more but recoups the loss by spending less time in the kitchen.

Finally, there's the matter of taste. The assumption in using a convenience food, whether we frame it clearly in our minds or not, is that it's going to taste good. Not quite up to the original, maybe, but close enough to strike the bargain.

But are the assumptions true? How much time do you actually save, and how much (more or less) do you actually pay? And how does the taste of the packaged product really stack up against its made-from-scratch version? We decided to put these questions to the test by comparing the preparation time and price of cake mix and homemade chocolate cake. And, by using panels of tasters (who would be uninformed about the source of the food they were eating), we'd also try for a consensus on taste appeal.

Our experiment focused on chocolate cake because it's an all-American standard that has highly popular packaged versions. For the mix we chose Duncan Hines Deluxe II Deep Chocolate, a big seller that was rated highly by Consumer Reports magazine last March. For the homemade cake, we went back to our tattered, hand-me-down copy of the 1930 Junior League Cookbook of Boise, Idaho, and chose Mrs. Gignay's Chocolate Cake, a recipe that's simple to make and that's proved its mettle for four generations.

To make things more interesting in the taste testing, we used three frostings. We adapted the homemade one from a deep-flavored blend in the 10th edition of the Fanny Farmer Cookbook. We also chose a canned frosting, Betty Crocker Creamy Deluxe Dark Dutch Fudge, and a packaged product, Betty Crocker Chocolate Frosting Mix. We baked two of each of our cakes--homemade and Duncan Hines--and applied the frostings so that we ended up with the following combinations: homemade cake with homemade frosting; homemade cake with canned frosting; Duncan Hines cake with homemade frosting; and Duncan Hines cake with packaged frosting mix.

First we made the cake from the mix, timing each step in the procedure. (We didn't include baking times in our calculations, because they were about the same for both cakes.) [TABLE OMITTED] Comparing the Time

Our homemade cake took 22 minutes with clean-up, versus 12 minutes for the mix, a difference of 10 minutes. Lesson: if you use a simple recipe and work efficiently, a homemade cake doesn't take much more time than one from a mix. The frosting was another story: 3 minutes for the powdered mix, versus 23 minutes for the homemade. Most of the difference lay not in extra steps or work, but in the unavoidably long heating and cooling processes. Comparing the Cost

The Duncan Hines cake mix cost $1.10 and the canned frosting $1.50, for a total of $2.60. (The packaged frosting mix, including the added butter, came to about the same price as the canned frosting.) By comparison, the homemade cake and frosting totalled $4.15, or about $1.50 more. It's not surprising that the expensive ingredients in the homemade cake and frosting, the ones that account for the higher cost--butter, chocolate and vanilla extract--are the ones you don't get in the mixes. (They substitute cheaper shortenings, cocoa and artificial flavor.) Moral: If you want the real thing, it will cost you. Comparing the Taste

Here's where we found the surprises. We administered the "blind" test to 31 people; the sexes were divided about evenly, and ages ranged mainly from the mid-20s to the late 50s. A form was used to record impressions and a questionnaire to get background information on the participants. The subjects were told they were testing four cake recipes. Actually, you'll recall, they were testing two cakes and three frostings, cross-mixed to produce four combinations. Each person ranked the "four cakes" in order of preference, with 4 the highest ranking and 1 the lowest, and also entered specific comments on the form. From the questionnaire, administered after the tasting was over, we derived information on age, sex, whether the subjects baked cakes themselves, and whether they used packaged mixes and why. Then, to try to discover the influence of family tradition, we asked whether their mothers had baked cakes, and whether from scratch or a mix. The forms and questionnaire were filled out anonymously.

Here are the key findings:

* There was little equivocation. People tended to have strong opinions and to back them up with specific comments. Those who preferred the packaged cake and frosting tended to characterize them as "moist" or "creamy" respectively, and to downgrade the homemade as "strong" or "dry." And those who preferred the homemade generally found a "lack of chocolate flavor" in the packaged cake and frostings.

* Strong as the individual opinions were, they varied greatly, with no cake a clear winner. In fact, the total score for any one cake differed by no more than a few percentage points from any other.

* Although instructed to differentiate in their minds between the frosting and the cake, many subjects had trouble making the separation. For example, the same subject might rate the Duncan Hines cake "too sweet" with one frosting and "less sweet" with another, which demonstrates that the palate, as much as we'd like to think otherwise, is no camera. It may not consistently show us the components of the image it records.

* We were especially interested in the effect of age. Would older people, their palates "imprinted" early in life with the virtues of homemade foods, tend to rank the homemade cake and frosting higher than younger people? No such pattern; the percentage of people over 40 who preferred the packaged cake and frosting mixes was about the same as for the under-40s. (Perhaps 40 was too young an age at which to draw the demarcation. Our oldest subject, an 87-year-old woman with a lifetime of baking experience, who wouldn't know a cake mix if it fell into her shopping basket, homed in without hesitation on the homemade cake-frosting combination. "This is my favorite," she said. "The other ones taste like wax.")

* We did find one strong correlation, and, in the end, it bears out a single truth: one likes what one is used to. Nearly all the people who indicated on the questionnaire that they never use cake mixes preferred the homemade cakes, and vice-versa; the people who said they use mixes "almost always" or "sometimes" tended to rate the mix-cakes highest.

What advice can we give from all this? Simple. If you like cake from mixes, it's probably best to stick with them. You may find the homemade cake and frosting recipes we used in our experiment too dry for your taste, more crumbly than you're used to, and too strong in chocolate flavor. On the other hand, if you prefer homemade but hesitate because you think it takes too long, think again. We took only 9 more minutes for the homemade cake recipe, and if you can manage to occupy yourself with something else for the 20 minutes it takes the homemade frosting to heat and cool, you may find the investment worth your while.

If the results of this experiment are representative, about half of us have developed a strong preference for the taste of cakes made with cocoa, artificial flavor and vegetable shortening, as opposed to those with chocolate, real vanilla and butter. And about half of us have apparently come to enjoy the consistency produced by propylene glycol monoesters, mono- and diglycerides, and cellulose gum, as well as the added visual impact of artificial coloring.

Is that discouraging? It depends how you look at it. We are preparing ourselves for space travel.