Apples and oranges

Apples and oranges is a common English idiom. It is used to describe unlike objects or people. One of the most well-known bits of popular wisdom in the English-speaking world is that apples and oranges cannot be compared. The ability to tell apples from oranges is learned.
The phrase is almost always used along with a warning that things in different categories cannot be compared, or that the comparison is improper.
History.
This idiom began as a comparison of "apples and oysters" in a book of proverbs published in 1670. This idiom has become a marker in English-speaking culture. Idioms are a common stumbling block for learners of a language.
Variants.
The idiom is not uniquely English.
In French-speaking Quebec, the idiom is "comparer des pommes avec des oranges" (to compare apples and oranges), In Europe, the French idiom is "comparer des pommes et des poires" (to compare apples and pears).
In Latin America, the Spanish idiom is "comparar papas y boniatos" (comparing potatoes and sweet potatoes) or "comparar peras con manzanas" (comparing pears and apples).
In British English, the phrase "chalk and cheese" means the same thing as "apples and oranges".
Comparison.
In order to compare anything, there needs to be a theory to be proven and framework for testing the theory. Three basic questions come first:
Scientific research.
The idiom has inspired scientific research projects. For example, the "British Medical Journal" published a study of red delicious apples compared with navel oranges. They were found to have many similarities.
There are differences in nutritional value.
Experiments at NASA Ames Research Center showed that apples and oranges decay at similar rates.