# Dynamic risk measure

In financial mathematics, a conditional risk measure is a random variable of the financial risk (particularly the downside risk) as if measured at some point in the future. A risk measure can be thought of as a conditional risk measure on the trivial sigma algebra.
A dynamic risk measure is a risk measure that deals with the question of how evaluations of risk at different times are related. It can be interpreted as a sequence of conditional risk measures.
A different approach to dynamic risk measurement has been suggested by Novak.

## Conditional risk measure

Consider a portfolio's returns at some terminal time $T$ as a random variable that is uniformly bounded, i.e., $X\in L^{\infty }\left({\mathcal {F}}_{T}\right)$ denotes the payoff of a portfolio. A mapping $\rho _{t}:L^{\infty }\left({\mathcal {F}}_{T}\right)\rightarrow L_{t}^{\infty }=L^{\infty }\left({\mathcal {F}}_{t}\right)$ is a conditional risk measure if it has the following properties for random portfolio returns $X,Y\in L^{\infty }\left({\mathcal {F}}_{T}\right)$:

### Conditional cash invariance

    $\forall m_{t}\in L_{t}^{\infty }:\;\rho _{t}(X+m_{t})=\rho _{t}(X)-m_{t}$

### Monotonicity

    $\mathrm {If} \;X\leq Y\;\mathrm {then} \;\rho _{t}(X)\geq \rho _{t}(Y)$

### Normalization

    $\rho _{t}(0)=0$
If it is a conditional convex risk measure then it will also have the property:

### Conditional convexity

    $\forall \lambda \in L_{t}^{\infty },0\leq \lambda \leq 1:\rho _{t}(\lambda X+(1-\lambda )Y)\leq \lambda \rho _{t}(X)+(1-\lambda )\rho _{t}(Y)$
A conditional coherent risk measure is a conditional convex risk measure that additionally satisfies:

### Conditional positive homogeneity

    $\forall \lambda \in L_{t}^{\infty },\lambda \geq 0:\rho _{t}(\lambda X)=\lambda \rho _{t}(X)$

## Acceptance set

The acceptance set at time $t$ associated with a conditional risk measure is
    $A_{t}=\{X\in L_{T}^{\infty }:\rho _{t}(X)\leq 0{\text{ a.s.}}\}$.
If you are given an acceptance set at time $t$ then the corresponding conditional risk measure is
    $\rho _{t}={\text{ess}}\inf\{Y\in L_{t}^{\infty }:X+Y\in A_{t}\}$
where ${\text{ess}}\inf$ is the essential infimum.

## Regular property

A conditional risk measure $\rho _{t}$ is said to be regular if for any $X\in L_{T}^{\infty }$ and $A\in {\mathcal {F}}_{t}$ then $\rho _{t}(1_{A}X)=1_{A}\rho _{t}(X)$ where $1_{A}$ is the indicator function on $A$. Any normalized conditional convex risk measure is regular.
The financial interpretation of this states that the conditional risk at some future node (i.e. $\rho _{t}(X)[\omega ]$) only depends on the possible states from that node. In a binomial model this would be akin to calculating the risk on the subtree branching off from the point in question.

## Time consistent property

A dynamic risk measure is time consistent if and only if $\rho _{t+1}(X)\leq \rho _{t+1}(Y)\Rightarrow \rho _{t}(X)\leq \rho _{t}(Y)\;\forall X,Y\in L^{0}({\mathcal {F}}_{T})$.

## Example: dynamic superhedging price

The dynamic superhedging price involves conditional risk measures of the form $\rho _{t}(-X)=\operatorname {*} {ess\sup }_{Q\in EMM}\mathbb {E} ^{Q}[X|{\mathcal {F}}_{t}]$.
It is shown that this is a time consistent risk measure.