Expected Value of a Random Variable - The Formal Way
In this post we'll try to explain the way the Expected Value of a Random Variable is formally defined.
It is assumed that notions like Random Variable, Sigma Algebra, Probability Measure, ... are known in a formal way.
Definition of the problem
A Random Variable is defined in a Measurable Probability Space $ (\Omega, F, P) $ where
$ \Omega $ = Universe Space
$ F $ = Sigma Algebra defined over $ \Omega $
$ P $ = Probability Measure
Its Expected Value is defined as follow
so now the problem becomes to find an operative definition for this integral.
It's important to notice that, at this point, the only well defined Integral we have is this one
This way we are able to associate a Probability Measure to each possible event in the Universe Space but we still don't know which way to integrate a Random Variable.
As a start we can make an assumption to make the job easier: let's assume that $ X $ is a Random Variable with finite cardinality, it is to say that the possible outcomes of $ X $ are finite or more formally
It implies that independently of the structure of $ \Omega $ it's possible to identify $ N $ Regions, each connected to a possible outcome.
More formally this concept could be expressed exploiting the fact that the Random Variable has been defined as an invertible function
So we can express the Random Variable just as a linear combination of indicator funcions related to each of the segments, multiplied by the real value associated to each region thus
At each of the regions in which the $ \Omega $ Universe Space has been segmented is associated a Measure of Probability as follows
With $ 1_{A_{k}} $ Indicator Function
It is a well defined integral.
This leads us to a well defined integral associated to the initial one
Let's notice that the result is equivalent to the definition of Expected Value for a Random Variable with finite cardinality that is usually presented in an intuitive way