# Feature Crosses: Check Your Understanding

Explore the options below.

Different cities in California have markedly different housing prices. Suppose you must create a model to predict housing prices. Which of the following sets of features or feature crosses could learn city-specific relationships between roomsPerPerson and housing price?
Three separate binned features: [binned latitude], [binned longitude], [binned roomsPerPerson]
Binning is good because it enables the model to learn nonlinear relationships within a single feature. However, a city exists in more than one dimension, so learning city-specific relationships requires crossing latitude and longitude.
One feature cross: [latitude X longitude X roomsPerPerson]
In this example, crossing real-valued features is not a good idea. Crossing the real value of, say, latitude with roomsPerPerson enables a 10% change in one feature (say, latitude) to be equivalent to a 10% change in the other feature (say, roomsPerPerson).
One feature cross: [binned latitude X binned longitude X binned roomsPerPerson]
Crossing binned latitude with binned longitude enables the model to learn city-specific effects of roomsPerPerson. Binning prevents a change in latitude producing the same result as a change in longitude. Depending on the granularity of the bins, this feature cross could learn city-specific or neighborhood-specific or even block-specific effects.
Two feature crosses: [binned latitude X binned roomsPerPerson] and [binned longitude X binned roomsPerPerson]
Binning is a good idea; however, a city is the conjunction of latitude and longitude, so separate feature crosses prevent the model from learning city-specific prices.