Multi-Objective Optimization
In contrast to Uni-Objective Optimization problems, in Multi-Objective Optimization problems there are multiple
An usual model is:
- $\text{opt} f_1(x), ..., f_q(x), x \in A$
- but usually in this case there is no single optimal solution - but a set of solutions where you cannot say which one is better
Example:
- suppose you want to buy a flat
- there are 2 criteria: cost and comfort
- you want to minimize the cost and maximize the comfort
-
- note that you cannot say that $d$ is better than $b$ or better than $a$
- but $c$ is clearly dominated by $b$: it's as comfortable as $c$, but cheaper
- the set of the best alternatives is called the Pareto-optimal set of alternatives
Multi-Criteria Problem
We have $q$ criteria and $n$ items
|
Criterion 1 |
Criterion 2 |
... |
Criterion $q$
|
Item 1 |
100 |
Medium |
... |
8
|
Item 2 |
100 |
Medium |
... |
8
|
... |
... |
... |
... |
...
|
Item $n$ |
55 |
Very Bad |
|
8
|
Goal: to rank the items
- there are lots of conflicting criteria (like price and comfort)
- there are different units and scales
- the single optimal solution does not exist
Instead of "Item" it can be "Action", "Alternative", etc
Formally we can write it as:
- objective: $\text{opt} z(x)$
- $z: \mathbb{R}^n \to \mathbb{R}^m$
- constraints: $g_i(x) \geqslant 0, x \in \mathbb{R}^n$
But it is possible to draw a direct parallel with Voting Theory!
|
Voter 1 |
Voter 2 |
... |
Voter $q$
|
Candidate 1 |
100 |
Medium |
... |
8
|
Candidate 2 |
100 |
Medium |
... |
8
|
... |
... |
... |
... |
...
|
Candidate $n$ |
55 |
Very Bad |
|
8
|
So these two problems are similar:
- Each voter ranks all candidates (alternatives)
- We apply some voting mechanism and find the global preference (the "best" alternative)
- All properties of Voting Theory are still available:
However there are differences:
- Not all criteria have the same weight
- in Voting Theory all votes are equally important
- here some criteria may be more important than others
- We need more information than just ranking
- There are different scales
- Since the scales can be numerical, we can compare the intensity of preference
Multi-Objective Optimization Problems
Choosing the Solution
Suppose we have obtained the Pareto-optimal set of solutions. How do we choose the "best" solution?
There are several approaches:
Also MCDA is used for that:
- find the Pareto-optimal solutions
- apply MCDA to find the best one
Sources