# Unconditional marginal probability

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#### Annakrutitskaya

##### Senior Member
Hello!

For a while, I meet the word "marginal" in a lot of texts on economics and/or math, but I have difficulty to understand it intuitively.
Margin is some sort of a limit, a border, over which something doesn't usually go.
Why then the unconditional probability is also called the marginal probability? What does margin have to do with the unconditionality?

I have just read a phrase in a math stats book: unconditional probability (aka marginal probability) refers to the probability of an event regardless of the past or future occurrence of other events.

Thank you very much.

• #### boozer

##### Senior Member
I have no idea what this is in mathematics, but in economics it is something like this:

For instance, you have the marginal propensity to consume. It is marginal because it refers to adding new income to the existing income and consuming a certain part of the new, added portion. For instance:
Income: 100 New income: 120
Consumption: 100 New consumption: 110
20 units have been added to your income, of which you have spent 10. Therefore, your marginal propensity to consume is 50%. The new addition of 20 is a margin to the main income. Every time a new margin is added, you will only spend 50% of it.

In fact, I believe Wiki explains something similar.
Marginal propensity to consume - Wikipedia

#### Edinburgher

##### Senior Member
In specialist jargon, particular combinations of words often have meanings that you may not expect them to have if you simply go by their individual usual meanings.
In statistics, "marginal probability" and "unconditional probability" are different names for the same thing. A good description (complete with "real-world example") is here:
Marginal distribution - Wikipedia
Perhaps there is a connection to the "margin" of the table that shows joint probability distribution.

#### Annakrutitskaya

##### Senior Member
I have no idea what this is in mathematics, but in economics it is something like this:

For instance, you have the marginal propensity to consume. It is marginal because it refers to adding new income to the existing income and consuming a certain part of the new, added portion. For instance:
Income: 100 New income: 120
Consumption: 100 New consumption: 110
20 units have been added to your income, of which you have spent 10. Therefore, your marginal propensity to consume is 50%. The new addition of 20 is a margin to the main income. Every time a new margin is added, you will only spend 50% of it.

In fact, I believe Wiki explains something similar.
Marginal propensity to consume - Wikipedia
Thank you very much. But no, this meaning is completely different in economics and in stats. That what has caused my confusion. In economics marginal means incremental, while in stats somehow used to mean unconditional probability.

#### Annakrutitskaya

##### Senior Member
In specialist jargon, particular combinations of words often have meanings that you may not expect them to have if you simply go by their individual usual meanings.
In statistics, "marginal probability" and "unconditional probability" are different names for the same thing. A good description (complete with "real-world example") is here:
Marginal distribution - Wikipedia
Perhaps there is a connection to the "margin" of the table that shows joint probability distribution.
Thank you very much. I see now - indeed it seems to refer to margins of the table.

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