Has this ever happened to you? You get a craving for a Hot n' Spicy Chicken Sammich With Hot n' Spicy Sauce from your local fast food joint. You go in, order the Hot n' Spicy Chicken Sammich With Hot n' Spicy Sauce, and a few minutes later walk to your table, sandwich in hand. You peel back the paper wrapper, take the sandwich up to your mouth and take that first bite. "Hmm...," you say to yourself, "this isn't all that spicy." You take a second bite, and as the Hot n' Spicy Sauce hits your tongue, you notice a vague sensation of something vaguely spicy in the corner of your mind reserved for tastes. One more bite, still nothing, just that vague sense of spiciness. Dissapointed, you finish the sandwich, and leave.
A week later, you're in a different part of town, and the craving for a Hot n' Spicy Chicken Sammich With Hot n' Spicy Sauce strikes again. You go into the local fast food place, same chain, different store, different clientele, and order the Hot n' Spicy Chicken Sammich With Hot n' Spicy Sauce. You peel back the paper wrapper, take the sandwich up to your mouth and take that first bite. You take another, and another - your tongue wallowing in near sadomasochistic pleasure of spiciness.
You wonder... why would the spiciness of a fast food item vary depending on the neighborhood? After all, the fast food chain is infamous for being able to provide the same basic food experience, whether in Brooklyn or Burma. It's all a matter of maximising sales for the region. Consider: BurgerJoint decides to start opening stores in India. Now, in India, a great deal of the population does not eat beef. Also, a great deal of the population are vegetarian. It is therefore in the best interest of the company to market food that will actually sell. Therefore you will see a large focus on vegetarian, and non-beef food products at an Indian BurgerJoint. If BurgerJoint can do this for a country, why not do it on a smaller scale?
To do this, they require data. Let's say BurgerJoint wants to sell a new Xtreme Hot n' Spicy Chicken Sammich With Xtreme Hot n' Spicy Sauce. They'll pick a city to test market the sandwich in, at a median level of spiciness. Feedback from sales will show that in many places the median level of spiciness is just fine, but in some areas the level of spiciness is too high, or too low. The regions with this level of feedback can be crossreferenced to various local factors: neighborhood ethnic makeup, income levels, etc - so that when the Xtreme Hot n' Spicy Chicken Sammich With Xtreme Hot n' Spicy Sauce goes national, neighborhoods that match the profile in the test city for hotter or milder Xtreme Hot n' Spicy Chicken Sammiches With Xtreme Hot n' Spicy Sauce will get the proper sandwich.
Simple, isn't it?