Monday, October 29, 2012

The Normal Distribution and Ergodicity

The normal distribution, which is represented by the familiar bell curve, is probably the most important probability distribution in statistics. The reason for this is the Central Limit Theorem, which holds that the sum of a large number of identically distributed but independent random outcomes will tend toward having a normal distribution as the number of included results increases, regardless of their original probability distribution.

For the purposes of finance, an important aspect of the Central Limit Theorem is that normal distributions will be ergodic, which is to say that they will exhibit the fractal quality of having indistinguishable characteristics regardless of at what scale they are viewed: a variable that moves with instantaneous, normally distributed perturbations will create a time path that looks the same whether you are looking at movements over one minute, one day, one month, or several years. The reason for this is that, as with every other sort of variable, sums of normally distributed variables tend to be normally distributed variables; the exception is that normally distributed variables also start that way.

To a very broad degree in finance, measurement time sequences are arbitrary. There is no reason to think that the "proper" span of time over which to consider results is a minute, a day, a month, or a year. There are some cosmological conditions -- such as the passing of night and day and the changing of the seasons -- and some social conditions -- such as regularly scheduled weekends and holidays during which financial and economic activity is restricted -- that will create regular cycles that might be taken into account for the purpose of measuring financial results, but beyond this there is no reason to think that any span of measurement is better than any other.

The usefulness of the normal distribution's ergodic property is so great that, in my estimation, it is often worthwhile to use the distribution even when it is known not to match experience for the purpose to which it is put.

Sunday, October 28, 2012

October 25, 2012 Trade and Update



Opened:
 Pos    Symbol                   Price     Comm       Net

 -2    NSC 22JUN13 50.0 P         1.35     0.02     (269.98)

Also on October 25, a $170.00 dividend was paid on 1,000 shares of GE.

October 23, 2012 Trade



Opened:
 Pos    Symbol                   Price     Comm       Net

 -3    HON 22JUN13 90.0 P         0.90     2.28     (267.72)

October 22, 2012 Trades

Opened:
 Pos    Symbol                   Price     Comm       Net
 -4    BMY 17NOV12 35.0 C         0.17     3.04      (64.96)

 -5    INTC 20NOV13 25.0 C        0.43     3.80     (211.20)

 -2    RIG 17NOV12 50.0 C         1.16     0.76     (231.24)

 -4    STX 17NOV12 27.0 C         2.00     3.05     (796.95)

Probability Distribution Functions and Random Variables

A probability distribution function (p.d.f.) is the collected measure of the likelihoods of each possible outcome of a random event. So a p.d.f. for the future price of a particular stock would give a likelihood of each possible future stock price, from zero on up.

Probability distribution functions have a couple qualities:
  1. they must be strictly non-negative; and
  2. the sum of all probabilities under a p.d.f. must be one.
If a random variable has a known p.d.f., two important values can be determined for it: its expected value, or mean, which is sometimes designated with the Greek letter μ; and variance, which is designated with σ². Variance is the expected value of the square of the difference between a random variable and its expected value.

There are a lot of things of interest about mean and variance, but for my purpose, only a couple are important.

First, the square root of variance, σ, or standard deviation, can be used as a measure of confidence intervals for a random variable: for a normally distributed variable, for example, the span within about 1.96 standard deviations of the mean of the variable forms a 95% confidence interval.

Second, both mean and variance are additive, which is to say that if X and Y are random variables, then generally the mean of X+Y is the mean of X plus the mean of Y, and the variance of X and Y is the variance of X plus the variance of Y. (The latter isn't quite true, because if X and Y are not independent -- the outcome of X affects the p.d.f. of Y -- then the variance of X+Y is the variance of X plus the variance of Y plus twice the covariance of X and Y. The covariance of two random variables is the expected value of the product of the differences between the variable and their respective means. I'll almost always be assuming independence between variables, so covariance won't matter to me much.)

In combination, these two factors create an important effect: the expected value for a sum of identically distributed independent variables grows with the number of variables included in the sum, while confidence intervals for this sum grow with the square root of the number of variables included in the sum. This gives the Law of Large Numbers: the average of a number of outcomes of independent variables from an identical distribution will approach their expected value.

Risk-Neutral Pricing

I have previously introduced the concept of the "risk premium" in investing, which holds that "risky" investments should have a higher expected rate of return than "non-risky" investments. In that earlier entry, I noted that Utility Theory provides a framework for explaining why the risk premium should exist. In this entry, I will try to explain a useful way of quantifying the disposition of risk premium: "risk-neutral pricing."

Typically, the risk premium will be spoken of in terms of "expected rate of return," as in, "the expected rate of return on common stocks is around 6%, while the expected rate of return on cash is around 1%." The higher expected rate of return stated here for stocks as opposed to cash indicates a higher risk premium for stocks.

However, when option securities are considered, this way of thinking runs aground. We might consider a call option, for example, that pays in dollars the amount (if any) by which a particular stock index exceeds a given value at close of trading on a particular day, or nothing if the stock index is less than or equal to the value. But now how do we come up with an expected rate of return for this option?

If we look at the Put-Call Parity equation,

     C - P = S - X * exp(-rT),

we see that on the right hand side of the equation are terms, S and X * exp(-rT), that should have the expected returns of stocks and of cash, respectively (cash being a risk-free asset, subject to the risk-free interest rate of r).

So what if we treat the call as the difference between the index value -- discounted at the expected returns on stocks -- and a riskless payment of the strike price -- discounted at the risk-free interest rate -- on the expiration date?

This method falls apart pretty quickly. Suppose we look at just a piece of the call using this perspective: the portion from where the index equals the strike price to where it's the strike price plus one. Let's suppose also that the strike price is 1000 and the expiration date is one year in the future. So the value of the stock piece, using the expected rates of return above, will be between 1000 and 1001 discounted by 6%, or between about 943.40 and 944.34. But the value of the risk-free piece will be 1000 discounted by 1%, or about 990.10. This suggests that the value of this portion of the call option will be between -46.70 and -45.76: negative values for something that can only ever result in positive cash flow! No rational person will give you money to accept the possibility that he might give you more money later on.

The way around this is to discount everything at the risk-free interest rate: both the stock portion and the strike price portion. This gives the value of the portion of the call option we're looking at as being between 0 and 0.99, which is very consistent with the value at the end of the year being between 0 and 1.

But where does the risk premium appear?

The risk premium appears in adjustments to the probabilities of different outcomes. In particular, prices will be determined using probabilities that are different from actual expectations.

In looking at the portion of the call option where the final index value is between the strike price and the strike price plus one, I never considered the probability of this outcome: I essentially took each potential outcome -- from the index being equal to the strike price up to being that value plus one -- as given, and looked at what the value of the option would be under that outcome. To get an actual value of the call option, I'd have to assign a probability to each of the outcomes, and generate a weighted average for the option's value under all possible outcomes. In essence, the risk premium on stocks is created because the pricing probability assumes that high returns on stocks are less likely than actual expectations.

So, why use the risk-less interest rate rather than some other rate?

First of all, it would probably be more accurate to use actual expectations, and then have a different expected rate of return apply to each possible outcome. But this would create a mess: how do you figure out what expected rate of return to apply to a given outcome, and from where (without a LOT of work) do you get actual expectations?

Risk-neutral pricing takes advantage of the fact that prices based on uncertain events are based on the product of the probability of given outcomes and the expected rate of return that applies to that outcome. So rather than (more accurately) using actual expectations for the probabilities and some yet-to-be-determined expected rate of return for each outcome, risk-neutral pricing uses a single expected rate of return (the risk-free interest rate) and adjusted probabilities to come up with the same price that would result from the theoretically more accurate method of using actual expectations and variable expected rates of return.

So, again, why use the risk-less interest rate rather than some other rate?

The class of securities that by definition earn the risk-free interest rate -- riskless securities -- place some constraints on the adjusted pricing probabilities: the price indicated, under whatever pricing algorithm we use, for a fixed amount paid under all possible outcomes must be consistent with the risk-free interest rate. If we try to do this using some other interest rate, we will have to have our adjusted probabilities sum to less than 1 if we use an interest rate lower than the risk-free interest rate, and we will have to have our adjusted probabilities sum to more than 1 if we use an interest rate higher than the risk-free rate. The adjusted probabilities would not be proper probability distribution functions, because a proper probability distribution function will sum to exactly 1. And, if a function that did not sum to 1 was used, it would create immediate arbitrage potential in either selling a payoff under all outcomes if the function sums to less than 1, or buying a payoff under all outcomes if the function sums to more than 1

So, it is ultimately simplicity that drives us to use a single interest rate and adjusted probabilities rather than variable expected rates of returns and actual expectations. Then, having limited ourselves in this way out of concern for simplicity, we are limited to using the risk-free interest rate as our single expected rate of return for the reason given above.

Finally, the reason this method is called "risk-neutral pricing" is that, using the adjusted pricing probabilities, all securities are assumed to have an equal expected rate of return, which is consistent with a risk-neutral market.

Friday, October 26, 2012

Put-Call Parity

The "Put-Call Parity" equation is an instance of the Law of One Price. Its mathematical form is

     C - P = S - X * exp(-rT),

where S is the price of a stock at time 0C is the price at time 0 of a European call option on S with strike price X and expiration time TP is the price at time 0 of a European put option on S with strike price X and expiration time T, and r is the risk-free interest rate (assumed to be constant).

The term X * exp(-rT)in the equation is constant, and it represents the value at time 0 of the amount X paid with certainty at time T. Figuring out the value of a call and a put with the given strike for any given price of the stock on the expiration date will show that being long such a call and short such a put will always have the save payoff at time T as the stock itself less a payment of X at the same instant, which invokes the Law of One Price.

Put-Call Parity is model independent, meaning that it will hold regardless of under what model prices are determined. Considered differently, any model that violates Put-Call Parity cannot be realistic.

Laws of Price: The Law of One Price

I think it's a useful exercise to try clearly to state as many requirements as possible that apply to efficient free market prices. So from time to time I'll devote a post to one or more of these, which I will call "Laws of Price."

The first that I'll look at is a very widely known law with many implementations: the Law of One Price.

Very simply, the Law of One Price states that if two different portfolios of securities have exactly the same payoff at a future date under all possible outcomes, then they must have the same price. Stated differently, there can be only one price for a given pattern of future cash flows, regardless of how the pattern is achieved.

So, a bet that pays $1 if the Steelers beat the Redskins this coming Sunday, plus a bet that pays $1 if the Redskins beat the Steelers in the same game, plus a bet that pays $1 in the event of a tie in the game, should have the same price as $1 paid after the game Sunday with certainty.* If they don't, then a guaranteed profit can be made by selling the more expensive position and buying the less expensive one: simple arbitrage. (The Law of One Price is sometimes referred to with the qualification "no arbitrage," or "arbitrage free.")

* -- These bets will not in fact add up to the value of $1 paid with certainty, but this is because the sports betting market is not efficient: the bookies and the odds-makers arrange to take a percentage of each bet as their compensation, so making all of these bets will cost more than $1 paid with certainty. Also, the "frictional costs" -- costs associated with transaction fees or expenses from acquiring or disposing of positions -- of attempting arbitrage will allow some deviation of prices away from a singular price. But the prices cannot stray too far apart.

Monday, October 22, 2012

October 19, 2012 Options Expirations, Trades, and Update


Closed:
 Pos    Symbol                   Price     Comm       Net
 -4 BMY 20OCT12 34.0 C            0.00     0.00        0.00
     Basis 09/24/2012:                              (102.48)
     Profit/(Loss):                                  102.48

 -2 RIG 20OCT12 50.0 C            0.00     0.00        0.00
     Basis 09/24/2012:                              (150.48)
     Profit/(Loss):                                  150.48

 -3 WAG 20OCT12 33.0 C            2.84     2.28      854.28
     Basis 05/21/2012:                              (457.72)
     Profit/(Loss):                                 (396.56)

     Value 08/31/2012:                              (940.89)
     Post-August Profit/(Loss):                       86.61

 -3 DD 20OCT12 40.0 P             0.00     0.00        0.00
     Basis 02/29/2012:                              (288.55)
     Profit/(Loss):                                  288.55

     Value 08/31/2012:                               (20.29)
     Post-August Profit/(Loss):                       20.29

 -5 INTC 20OCT12 25.0 P           3.735    0.00    1,867.50
     Basis 08/17/2012:                              (242.70)
     Profit/(Loss):                               (1,624.80) 

     Value 08/31/2012:                              (494.90)
     Post-August Profit/(Loss):                     (494.90)

 -3 SNDK 20OCT12 35.0 P           0.00     0.00        0.00
     Basis 02/21/2012:                              (554.08)
     Profit/(Loss):                                  554.08

     Value 08/31/2012:                              (164.04)
     Post-August Profit/(Loss):                      164.04

 -4 STX 20OCT12 28.0 P            0.10     0.00       40.00
     Basis 08/21/2012:                              (208.96)
     Profit/(Loss):                                  168.96

 -2 TM 20OCT12 55.0 P             0.00     0.00        0.00
     Basis 02/29/2012:                              (109.39)
     Profit/(Loss):                                  109.39

     Value 08/31/2012:                                (2.47)
     Post-August Profit/(Loss):                        2.47

 -3 UPS 20OCT12 33.0 P            0.00     0.00        0.00
          Basis 02/23/2012:                              (249.39)
     Profit/(Loss):                                  249.39

     Value 08/31/2012:                               (12.47)
     Post-August Profit/(Loss):                       12.47


Opened:
 Pos    Symbol                   Price     Comm       Net
 500 INTC INTEL CORPORATION      21.265    0.00   10,632.50

 400 STX SEAGATE TECH PLC        27.90     0.00   11,160.00

 -4 BBBY 20OCT12 40.0 P           0.50     1.14     (148.86)

Also, on October 3 I withdrew $2,500.00 from the account and a dividend of $136.00 became payable November 1 on 400 shares of BMY; on October 5 a dividend of $206.00 became payable November 1 on 400 shares of VZ; and on October 17 a dividend of $112.40 became payable November 15 on 200 shares of PG.

October 2, 2012 Corporate Action



Closed:
 Pos    Symbol                   Price     Comm       Net
 34    DYN                        0.03     0.00        0.11


Opened:
 Pos    Symbol                   Price     Comm       Net
 -4    DYN 02OCT17 40.0 C         0.00     0.00        0.00

This is basically the net result for shareholders of Dynegy's agreement to emerge from bankruptcy.

Sunday, October 7, 2012

An Options Quiz I

1.  Which day of the month is normally the last day on which standard equity option contracts can be traded?

a) The 20th.
b) The first Wednesday after the 15th of the month.
c) The third Friday of the month.
d) The Friday immediately before the third Saturday of the month.
e) The last Friday of the month.

2. Which event will generally cause the price of a call option to increase?

a) The passage of time.
b) An increase in the implied volatility of the contract.
c) A drop in the price of the underlying security.
d) An increase in the risk-free interest rate.
e) None of the above.

3. If the market for a security is efficient, which of the following should never occur?

a) Expected rate of return on the security is less than the risk-free interest rate.
b) Price of the security doubles in less than one month.
c) A portfolio with an identical cash flow can be created with a lower total price.
d) Highest bid for the security is greater than the lowest ask.
e) Both (c) and (d).

4. A European-style options differs from an American-style options in what way?

a) European-style options can only be exercised on their date of expiration, while American-style options can be exercised on any date prior to their date of expiration.
b) European-style options can be exercised on any date prior to their date of expiration, while American-style options can only be exercised on their date of expiration.
c) European-style options are traded in euros, while American-style options are traded in dollars.
d) European-style options are only traded in increments of five cents, while American-style options are traded in one cent increments.
e) European-style options are traded on commodities, currencies, and interest rates, while American-style options are traded only on corporate equity.

5. Options differ from a futures in what way?

a) Futures can be held as investments, while options can only be used for trading.
b) Short positions cannot be held on options, but they can on futures.
c) "Futures" and "options" are just different names for the same thing.
d) Futures can only be traded on very large positions in the underlying security or commodity, while options can be traded on relatively small amounts.
e) Futures are always settled at expiration, but options are settled only at the owner's discretion.

6. If on the 1st of October, the exchange-traded option contracts available for a particular stock have expiration months of October and November; January, February, and May of the following year; and January of the second following year, then on the 1st of November the following expiration months will probably be available for exchange-traded options contracts on the stock:

a) October, November, and December; January, February, and May of the following year; and January of the second following year.
b) November; January, February, and May of the following year; and January of the second following year.
c) November and December; January, February, March, and May of the following year; and January of the second following year.
d) November and December; January, February, and May of the following year; and January of the second following year.
e) November and December; January, February, May, and August of the following year; and January of the second following year.

Answers:

1(c), 2(b), 3(e), 4(a), 5(e), 6(d)