Raising the bar

Vector autoregression (VAR)

Suppose we are observing two stocks and their respective returns are To take into account their interdependence, we consider a vector autoregression

(1)

Try to repeat for this system the analysis from Section 3.5 (Application to an AR(1) process) of the Guide by A. Patton and you will see that the difficulties are insurmountable. However, matrix algebra allows one to overcome them, with proper adjustment.

Problem

A) Write this system in a vector format

(2)

What should be in this representation?

B) Assume that the error in (1) satisfies

(3) for with some symmetric matrix

What does this assumption mean in terms of the components of from (2)? What is if the errors in (1) satisfy

(4) for for all

C) Suppose (1) is stationary. The stationarity condition is expressed in terms of eigenvalues of but we don't need it. However, we need its implication:

(5) .

Find

D) Find

E) Find

F) Find

G) Find

Solution

A) It takes some practice to see that with the notation

the system (1) becomes (2).

B) The equations in (3) look like this:

Equalities of matrices are understood element-wise, so we get a series of scalar equations for

Conversely, the scalar equations from (4) give

for .

C) (2) implies or by stationarity or Hence (5) implies

D) From (2) we see that depends only on (information set at time ). Therefore by the LIE

E) Using the previous post

(by stationarity and (3)). Thus, and (see previous post).

F) Using the previous result we have

G) Similarly,

Autocorrelations require a little more effort and I leave them out.

Vector autoregressions: preliminaries

Suppose we are observing two stocks and their respective returns are A vector autoregression for the pair is one way to take into account their interdependence. This theory is undeservedly omitted from the Guide by A. Patton.

Required minimum in matrix algebra

Matrix notation and summation are very simple.

Matrix multiplication is a little more complex. Make sure to read Global idea 2 and the compatibility rule.

The general approach to study matrices is to compare them to numbers. Here you see the first big No: matrices do not commute, that is, in general

The idea behind matrix inversion is pretty simple: we want an analog of the property that holds for numbers.

Some facts about determinants have very complicated proofs and it is best to stay away from them. But a couple of ideas should be clear from the very beginning. Determinants are defined only for square matrices. The relationship of determinants to matrix invertibility explains the role of determinants. If is square, it is invertible if and only if (this is an equivalent of the condition for numbers).

Here is an illustration of how determinants are used. Suppose we need to solve the equation for where and are known. Assuming that we can premultiply the equation by to obtain (Because of lack of commutativity, we need to keep the order of the factors). Using intuitive properties and we obtain the solution: In particular, we see that if then the equation has a unique solution

Let be a square matrix and let be two vectors. are assumed to be known and is unknown. We want to check that solves the equation (Note that for this equation the trick used to solve does not work.) Just plug

(write out a couple of first terms in the sums if summation signs frighten you).

Transposition is a geometrically simple operation. We need only the property

Variance and covariance

Property 1. Variance of a random vector and covariance of two random vectors are defined by

respectively.

Note that when variance becomes

Property 2. Let be random vectors and suppose are constant matrices. We want an analog of In the next calculation we have to remember that the multiplication order cannot be changed.

(applying )

Solution to Question 1 from UoL exam 2020

The assessment was an open-book take-home online assessment with a 24-hour window. No attempt was made to prevent cheating, except a warning, which was pretty realistic. Before an exam it's a good idea to see my checklist.

Question 1. Consider the following ARMA(1,1) process:

(1)

where is a zero-mean white noise process with variance , and assume and , which together make sure is covariance stationary.

(a) [20 marks] Calculate the conditional and unconditional means of , that is, and

(b) [20 marks] Set . Derive the autocovariance and autocorrelation function of this process for all lags as functions of the parameters and .

(c) [30 marks] Assume now . Calculate the conditional and unconditional variances of that is, and

Hint: for the unconditional variance, you might want to start by deriving the unconditional covariance between the variable and the innovation term, i.e.,

(d) [30 marks] Derive the autocovariance and autocorrelation for lags of 1 and 2 as functions of the parameters of the model.

Hint: use the hint of part (c).

Solution

Part (a)

Reminder: The definition of a zero-mean white noise process is

(2) for all and for all

A variable indexed is known at moment and at all later moments and behaves like a constant for conditioning at such moments.

Moment is future relative to   The future is unpredictable and the best guess about the future error is zero.

The recurrent relationship in (1) shows that

(3) does not depend on the information that arrives at time and later.

Hence, using also linearity of conditional means,

(4)

The law of iterated expectations (LIE): application of based on information available at time and subsequent application of based on no information, gives the same result as application of

Since is covariance stationary, its means across times are the same, so and

Part (b)

With we get and from part (a) Using (2), we find variance

and first autocovariance

(5)

Second and higher autocovariances are zero because the subscripts of epsilons don't overlap.

Autocorrelation function: (this is always true),

for

This is characteristic of MA processes: their autocorrelations are zero starting from some point.

Part (c)

If we replace all expectations in the definition of variance, we obtain the definition of conditional variance. From (1) and (4)

By the law of total variance

(6)

(an additive constant does not affect variance)

By the LIE and (3)

Here so

(7)

This equation leads to

and, finally,

(8)

Part (d)

From (7)

(9)

It follows that

(a constant is not correlated with anything)

From (7) and from (9)

From (3)

Using also the white noise properties and stationarity of

we are left with

Hence,

and using (8)

The finish is close.

This simplifies to

(10)

By (7)

Finally, using (10)

A couple of errors have been corrected on June 22, 2021. Hope this is final.

Put debit spread

This post parallels the one about the call debit spread. A combination of several options in one trade is called a strategy. Here we discuss a strategy called a put debit spread. The word "debit" in this name means that a trader has to pay for it. The rule of thumb is that if it is a debit (you pay for a strategy), then it is less risky than if it is a credit (you are paid). Let denote the price of the put with the strike suppressing all other variables that influence the put price.

Assumption. The market values higher events of higher probability. This is true if investors are rational and the market correctly reconciles views of different investors.

We need the following property: if are two strike prices, then for the corresponding put prices (with the same expiration and underlying asset) one has

Proof.  A put price is higher if the probability of it being in the money at expiration is higher. Let be the stock price at expiration Since is a moment in the future, is a random variable. For a given strike the put is said to be in the money at expiration if If and then It follows that the set is a subset of the set Hence the probability of the event is higher than that of the event and

Put debit spread strategy. Select two strikes buy (take a long position) and sell (take a short position). You pay for this.

Our purpose is to derive the payoff for this strategy. We remember that if then the put expires worthless.

Case In this case both options expire worthless and the payoff is the initial outlay: payoff

Case Exercising the put , in comparison with selling the stock at the market price you gain The second option expires worthless. The payoff is: payoff

Case Both options are exercised. The gain from is, as above, The holder of the long put sells you stock at price Since your position is short, you have nothing to do but comply. The alternative would be to buy at the market price, so you lose The payoff is: payoff

Summarizing, we get:

payoff

Normally, the strikes are chosen so that From the payoff expression we see then that the maximum profit is the maximum loss is and the breakeven stock price is This is illustrated in Figure 1, where the stock price at expiration is on the horizontal axis.

Figure 1. Payoff from put debit spread. Source: https://www.optionsbro.com/

Conclusion. For the strategy to be profitable, the price at expiration should satisfy Buying a put debit spread is appropriate when the price is expected to stay in that range.

In comparison with the long put position taking at the same time the short call position allows one to reduce the initial outlay. This is especially important when the stock volatility is high, resulting in a high put price. In the difference that volatility component partially cancels out.

Remark. There is an important issue of choosing the strikes. Let denote the stock price now. The payoff expression allows us to rank the next choices in the order of increasing risk: 1) (both options are in the money, less risk), 2) and 3) (both options are out of the money, highest risk).  Also remember that a put debit spread is less expensive than buying and selling in two separate transactions.

Exercise. Analyze a put credit spread, in which you sell and buy .

Call debit spread

A combination of several options in one trade is called a strategy. Here we discuss a strategy called a call debit spread. The word "debit" in this name means that a trader has to pay for it. The rule of thumb is that if it is a debit (you pay for a strategy), then it is less risky than if it is a credit (you are paid). Let denote the call price with the strike suppressing all other variables that influence the call price.

Assumption. The market values higher events of higher probability. This is true if investors are rational and the market correctly reconciles views of different investors.

We need the following property: if are two strike prices, then for the corresponding call prices (with the same expiration and underlying asset) one has

Proof.  A call price is higher if the probability of it being in the money at expiration is higher. Let be the stock price at expiration Since is a moment in the future, is a random variable. For a given strike the call is said to be in the money at expiration if If and then It follows that the set is a subset of the set Hence the probability of the event is lower than that of the event and

Call debit spread strategy. Select two strikes buy (take a long position) and sell (take a short position). You pay for this.

Our purpose is to derive the payoff for this strategy. We remember that if then the call expires worthless.

Case In this case both options expire worthless and the payoff is the initial outlay: payoff

Case Exercising the call and immediately selling the stock at the market price you gain The second option expires worthless. The payoff is: payoff (In fact, you are assigned stock and selling it is up to you).

Case Both options are exercised. The gain from is, as above, The holder of the long call buys from you at price Since your position is short, you have nothing to do but comply. You buy at and sell at Thus the loss from is The payoff is: payoff

Summarizing, we get:

payoff

Normally, the strikes are chosen so that From the payoff expression we see then that the maximum profit is the maximum loss is and the breakeven stock price is This is illustrated in Figure 1, where the stock price at expiration is on the horizontal axis.

Figure 1. Payoff for call debit strategy. Source: https://www.optionsbro.com/

Conclusion. For the strategy to be profitable, the price at expiration should satisfy Buying a call debit spread is appropriate when the price is expected to stay in that range.

In comparison with the long call position taking at the same time the short call position allows one to reduce the initial outlay. This is especially important when the stock volatility is high, resulting in a high call price. In the difference that volatility component partially cancels out.

Remark. There is an important issue of choosing the strikes. Let denote the stock price now. The payoff expression allows us to rank the next choices in the order of increasing risk: 1) (both options are in the money, less risk), 2) and 3) (both options are out of the money, highest risk).  Also remember that a call debit spread is less expensive than buying and selling in two separate transactions.

Exercise. Analyze a call credit spread, in which you sell and buy .

Solution to Question 2 from UoL exam 2018, Zone B

There are three companies, called A, B, and C, and each has a 4% chance of going bankrupt. The event that one of the three companies will go bankrupt is independent of the event that any other company will go bankrupt.

Company A has outstanding bonds, and a bond will have a net return of if the corporation does not go bankrupt, but it will have a net return of , i.e., losing everything invested, if it goes bankrupt. Suppose an investor buys $1000 worth of bonds of company A, which we will refer to as portfolio .

Suppose also that there exists a security whose payout depends on the bankruptcy of companies B and C in a joint fashion. In particular, if neither B nor C go bankrupt, this derivative will have a net return of . If exactly one of B or C go bankrupt, it will have a net return of , i.e., losing half of the investment. If both B and C go bankrupt, it will have a net return of , i.e., losing the whole investment. Suppose an investor buys $1000 worth of this derivative, which is then called portfolio .

(a) Calculate the VaR at the critical level for portfolios and . [30 marks]

Independence of events. Denote the events that company A goes bankrupt and does not go bankrupt, resp. A similar notation will be used for the other two companies. The simple definition of independence of bankruptcy events would be too difficult to apply to prove independence of all events that we need. A general definition of independence of variables is that their sigma-fields are independent (it will not be explained here). This general definition implies that in all cases below we can use multiplicativity of probability such as

The events here have a simple interpretation: the first is that “both B and C fail”, the second is “both B and C fail”, and the third is that “either (B fails and C does not) or (B does not fail and C does)” (they do not intersect and additivity of probability applies).

Let be returns on A and the security S, resp. From the problem statement it follows that these returns are described by the tables
Table 1

	Prob
0	0.96
-100	0.04

Table 2

	Prob
0	0.9216
-50	0.0768
-100	0.0016

Everywhere we will be working with percentages, so the dollar values don’t matter.

From Table 1 we conclude that the distribution function of return on A looks as follows:

Figure 1. Distribution function of portfolio A

At the function jumps up by 0.04, at by another 0.96. The dashed line at is used in the definition of the VaR using the generalized inverse:

From Table 2 we see that the distribution function of return on S looks like this:

The first jump is at , the second at and third one at . As above, it follows that

(b) Calculate the VaR at the critical level for the joint portfolio . [20 marks]

To find the return distribution for , we have to consider all pairs of events from Tables 1 and 2 using independence.

1.

2.

3.

4.

5.

6.

Since we deal with a joint portfolio, percentages for separate portfolios should be translated into ones for the whole portfolio. For example, the loss of 100% on one portfolio and 0% on the other means 50% on the joint portfolio (investments are equal). There are two such losses, in lines 2 and 5, so the probabilities should be added. Thus, we obtain the table for the return on the joint portfolio:

Table 3

	Prob
0	0.884736
-25	0.073728
-50	0.0384
-75	0.003072
-100	0.000064

Here only the first probability exceeds 0.1, so the definition of the generalized inverse gives

(c) Is VaR sub-additive in this example? Explain why the absence of sub-additivity may be a concern for risk managers. [20 marks]

To check sub-additivity, we need to pass to positive numbers, as explained in other posts. Zeros remain zeros, the inequality is true, so sub-additivity holds in this example. Lack of sub-additivity is an undesirable property for risk managers, because for them keeping the VaR at low levels for portfolio parts doesn’t mean having low VaR for the whole portfolio.

(d) The expected shortfall at the critical level can be defined as

where is a return or dollar amount. Calculate the expected shortfall at the critical level for portfolio . Is this risk measure sub-additive? [30 marks]

Using the definition of conditional expectation and Table 3, we have (the time subscript can be omitted because the problem is static)

There is a theoretical property that the expected shortfall is sub-additive.

Solution to Question 2 from UoL exam 2019, zone B

Suppose the parameters in a GARCH (1,1) model

   (1)

are , the index refers to days and is zero-mean white noise with conditional variance .

(a) What are the requirements for this process to be covariance stationary, and are they satisfied here? [20 marks]

If the coefficients satisfy the condition for positivity, , then the condition for covariance-stationarity is . They are barely satisfied.

(b) What is the long-run average volatility? [20 marks]

We use the facts that for all t. Applying the unconditional mean to regression (1) and using the LIE we get

and

.

(c) If the current volatility is 2.5% per day, what is your estimate of the volatility in 20, 40, and 60 days? [20 marks]

On p.107 of the Guide there is the derivation of the equation

    (2)

I gave you a slightly easier derivation in my class, please use that one. If we interpret "current" as and "in twenty days" as , then

For use the same formula to get 0.016692, 0.013725, resp. I did it in Excel and don't envy you if you have to do it during an exam.

(d) Suppose that there is an event that decreases the current volatility by 1.5%to 1% per day. Estimate the effect on the volatility in 20, 40, and 60 days. [20 marks]

Calculations are the same, just replace 0.025 by 0.01. Alternatively, one can see that the previous values will go down by , which results in volatility values 0.012146, 0.009934 and 0.008125.

(e) Explain what volatility should be used to price 20-, 40-and 60-day options, and explain how you would calculate the values. [20 marks]

The only unobservable input to the Black-Scholes option pricing formula is the stock price volatility. In the derivation of the formula the volatility is assumed to be constant. The value of the constant should depend on the forecast horizon. If we, say, forecast 20 days ahead, we should use a constant value for all 20 days. This constant can be obtained as an average of daily forecasts obtained from the GARCH model.

If the GARCH is not used, a simpler approach is applied. If the average daily volatility is , then assuming independent returns, over a period of days volatility is .

In practice, traders go back from option prices to volatility. That is, they use observed option prices to solve the Black-Scholes formula for volatility (find the root of an equation with the price given). The resulting value is called implied volatility. If it is plugged back into the Black-Scholes formula, the observed option price will result.

Solution to Question 3 from UoL exam 2019, zone A

(a) Define the concept of trade duration in financial markets and explain briefly why this concept is economically useful. What features do trade durations typically exhibit and how can we model these features? [25 marks]

High frequency traders (HFT) may trade every millisecond. Orders from traders arrive at random moments and therefore the trade times are not evenly spaced. It makes sense to model the differences

between transaction times. (The Guide talks about differences between times of returns but I don’t like this because on small time frames people are interested in prices, not returns.) Those differences are called durations. They are economically interesting because 1) they tell us something about liquidity: periods of intense trading are generally periods of greater market liquidity than periods of sparse trading (there is also after-hours trading between 16:00 and 20:30, New York time, when trading may be intense but liquidity is low) and 2) durations relate directly to news arrivals and the adjustment of prices to news, and so have some use in discussions of market efficiency.

The trading session in the USA is from 9:30 to 16:00, New York time. Durations exhibit diurnality (that is, intraday seasonality): transactions are more frequent (durations are shorter) in the first and last hour of the trading session and less frequent around lunch, see Figure 16.6 from the Guide.

Higher frequency in the first hour results from traders rebalancing their portfolios after overnight news and in the last hour – from anticipation of news during the next night.

The main decomposition of durations is

so

In the first equation is the diurnal component and is called a de-seasonalized duration (it has not been defined here). The second follows from the first.

I am not sure that you need the third equation. The fourth equation is used below. In the third equation is regressed on dummies of half-hour periods (there are 13 of them in the trading session; the constant is not included to avoid the dummy trap). In the fourth equation it is regressed on the first and second power of the time variable , which measures time in hours starting from the previous midnight. This is called a polynomial regression. Both regressions can capture diurnality.

(b) Describe the Engle and Russell (1998) autoregressive conditional duration (ACD) model. [25 marks]

Instead of the duration model considered in part (a) Engle and Russell suggest the ACD model

where

   (1)

The first decomposition is the same as above. The second equation decomposes the de-seasonalized duration into a product of deterministic and stochastic components. To understand the idea, we can compare (1) with the GARCH(1,1) model:

   (2)

Equations (1) and (2) are similar. The assumptions about the random components are different: in (1) we have , in (2) . This is because in (2) the epsilons are deviations from the mean and may change sign; in (1) the epsilons come from durations and should be positive. To obtain the last equation in (1) from the GARCH(1,1) in (2) one has to make replacements

. (3)

This is important to know, to understand the comparison of the ML method for the two models below.

(c) Compare the conditions for covariance stationarity, identification and positivity of the duration series for the ACD(1,1) to those for the GARCH(1,1). [25 marks]

Those conditions for GARCH are

Condition 1: for positive variance,

Condition 2: if for identification,

Condition 3: for covariance stationarity.

For ACD they are the same, because both are essentially ARMA models.

(d) Illustrate the relationship between the log-likelihood of the ACD(1,1) model and the estimation of a GARCH(1,1) model using the normal likelihood function. [25 marks]

Because of the assumption in (1) we cannot use the normal distribution for (1). Instead the exponential random variable is used. It takes only positive values; its density is zero on the left half-axis and is an exponential function on the right half-axis:

so and

Here is a positive number and is the density. We take as required by the ACD model. This implies so is distributed as Its density is

The rest is logical: plug from the ACD model (1), then take log and then add those logs to obtain the log-likelihood. A. Patton gives the log-likelihood for GARCH, whose derivation I could not find in the book. But from (3) we know that there should be similarity after replacement . To this Patton adds that the GARCH likelihood is simply a linear transformation of the ACD likelihood.

FN3142 Chapter 14 Risk management and Value-at-Risk: Backtesting

Here I added three videos and corresponding pdf files on three topics:

Chapter 14. Part 1. Evaluating VAR forecasts

Chapter 14. Part 2. Conditional coverage tests

Chapter 14. Part 3. Diebold-Mariano test for VaR forecasts

Be sure to watch all of them because sometimes I make errors and correct them in later videos.

All files are here.

Unconditional coverage test

FN3142 Chapter 13. Risk management and Value-at-Risk: Models

Chapter 13 is divided into 5 parts. For each part, there is a video with the supporting pdf file. Both have been created in Notability using an iPad. All files are here.

Part 1. Distribution function with two examples and generalized inverse function.

Part 2. Value-at-Risk definition

Part 3. Empirical distribution function and its estimation

Part 4. Models based on flexible distributions

Part 5. Semiparametric models, nonparametric estimation of densities and historical simulation.

Besides, in the subchapter named Expected shortfall you can find additional information. It is not in the guide but it was required by one of the past UoL exams.