A blog on financial markets and their regulation
Bayesians in finance redux
March 7, 2010Posted by on
In November last year, I wrote a brief post
about Bayesians in finance. The post was brief because I thought that
what I was saying was obvious. A long and inconclusive exchange with
Naveen in the comments section of another
post has convinced me that a much longer post is called for. The
Bayesian approach is perhaps not as obvious as I assumed.
When finance professors walk into a classroom, they want to build
on what the statistics professors have covered in their courses. When
I am teaching portfolio theory, I do not want to spend half an hour
explaining the meaning of covariance; I would like to assume that the
statistics professor has already done that. That is how division of labour is supposed to work in a
pin factory or in a university.
Unfortunately, there is a problem with this division of labour
– most statistics professors teach classical statistics. That is
true even of those statisticians who prefer Bayesian techniques in
their research work! The result is that many finance students wrongly
think that when the finance professors talk of expected returns,
variances and betas, they are referring to the classical concepts
grounded in relative frequencies. Worse still, some students think
that the means and covariances used in finance are sample means and
sample covariances and not the population means and covariances.
In business schools like mine where the case method dominates the
pedagogy, these errors are probably less (or at least do less damage)
because in the case context, the need for judgemental estimates for
almost everything of interest becomes painfully obvious to the
students. The certainties of classical statistics dissolve into utter
confusion when confronted with messy “case facts”, and
this is entirely a good thing.
But if cases are not used or used sparingly, and the statistics
courses are predominantly classical, there is a very serious danger
that finance students end up thinking of the probability concepts in
finance in classical relative frequency terms.
Nothing could be farther from the truth. To see how differently
finance theory looks at these things, it is instructive to go back to
some of the key papers that established and developed modern portfolio
theory over the years.
Here is how Markowitz begins his Nobel prize winning paper
(“Portfolio Selection”, Journal of Finance, 1952) more
than half a century ago:
The process of selecting a portfolio may be divided into two stages.
The first stage starts with observation and experience and ends with
beliefs about the future performances of available securities. The
second stage starts with the relevant beliefs about future performances
and ends with the choice of portfolio.
Many finance students would probably be astonished to read words
like observation, experience, and beliefs instead of terms like
historical data and maximum likelihood estimates. This was the paper
that gave birth to modern portfolio theory and there is no doubt in
Markowitz’ mind that the probability distributions (and the
means, variances and covariances) are subjective beliefs and not
classical relative frequencies.
Markowitz is also crystal clear that what matters is not the
historical data but beliefs about the future – historical data
is of interest only in so far as it helps form those beliefs about
the future. He also seems to take it for granted that different people
will have different beliefs. He is helping each individual solve his
or her portfolio problem and is not bothered about how these choices
affect the equilibrium prices in the market.
When William Sharpe developed the Capital Asset Pricing Model that
won him the Nobel prize, he was trying to determine the market equilibrium
and he had to assume that all investors have the same beliefs but did
so with great reluctance:
… we assume homogeneity of investor expectations: investors are
assumed to agree on the prospects of various investments – the
expected values, standard deviations and correlation coefficients
described in Part II. Needless to say, these are highly restrictive
and undoubtedly unrealistic assumptions. However, … it is far from
clear that this formulation should be rejected – especially in
view of the dearth of alternative models
But finance theory quickly went back to the idea that investors had
different beliefs. Treynor and Black (“How to use security
analysis to improve portfolio selection,” Journal of
Business, 1973) interpreted the CAPM as saying
…in the absence of insight generating expectations different from
the market consensus, the investor should hold a replica of the market
Treynor and Black devised an elegant model of portfolio choice
when investors had out of consensus beliefs.
The viewpoint in this paper is that of an individual investor who
is attempting to trade profitably on the diiference between his
expectations and those of a monolithic market so large in relation to
his own trading that market prices are unaffected by it.
Similar ideas can be seen in the popular Black Litterman model
(“Global Portfolio Optimization,” Financial Analysts
Journal, September-October 1992). Black and Litterman started
with the following postulates:
- We believe there are two distinct sources of information about
future excess returns – investor views and market equilibrium.
- We assume that both sources of information are uncertain and are
best expressed as probability distributions.
- We choose expected excess returns that are as consistent as
possible with both sources of information.
Even if we stick to the market consensus, the CAPM beta itself has
to be interpreted with care. The derivation of the CAPM makes it clear
that the beta is actually the ratio of a covariance to a variance and
both of these are parameters of the subjective probability
distribution that defines the market consensus. Statisticians
instantly recognize that the ratio of a covariance to a variance is
identical to the formula for a regression coefficient and are tempted
to reinterpret the beta as such.
This may be formally correct, but it is misleading because it
suggests that the beta is defined in terms of a regression on past
data. That is not the conceptual meaning of beta at all. Rosenberg and
Guy explained the true meaning of beta very elegantly in their paper
(“Prediction of beta from investment fundamentals”,
Financial Analysts Journal, 1976) introducing what are
now called fundamental betas:
It is instructive to reach a judgement about beta by carrying out an
imaginary experiment as follows. One can imagine all the various
events in the economy that may occur, and attempt to answer in each
case the two questions: (l) What would be the security return as a
result of that event? and (2) What would be the market return as a
result of that event?
This approach is conceptually revealing but is not always practical
(though if you are willing to spend enough money, you can access the
fundamental betas computed by firms like Barra which Barr Rosenberg
founded and later left). In practice, our subjective belief about the
true beta of a company involves at least the following inputs:
- The beta is equal to unity unless there is enough reason to
believe otherwise. The value of unity (the beta of an average stock)
provides an important anchor which must be taken into account even
when there is other evidence. It is not uncommon to find that simply
equating beta to unity outperforms the beta estimated by naive
- What this means is that betas obtained by other means must be
shrunk towards unity. An estimated beta exceeding one must be reduced
and an estimated beta below one must be increased. One can do this
through a formal Bayesian process (for example, by using a Bayes-Stein
shrinkage estimator), or one can do it purely subjectively based on
the confidence that one has in the original estimate.
- The beta depends on the industry to which the firm belongs. Since
portfolio betas can be estimated more accurately than individual
betas, this is often the most important input into arriving at a
judgement about the true beta of a company.
- The beta depends on the leverage of the company and if the
leverage of the company is significantly different from that of the
rest of the industry, this needs to be taken into account by
unlevering and relevering the beta.
- The beta estimated by regressing the returns of the stock on the
market over different time periods provides useful information about
the beta provided the business mix and the leverage have not changed
too much over the sample period. Since this assumption usually precludes very
long sample periods, the beta estimated through this route typically
has a large confidence band and becomes meaningful only when combined
with the other inputs.
- Subjective beliefs about possible future changes in the beta
because of changing business strategy or financial strategy must also
be taken into account.
Much of the above discussion is valid for estimating Fama-French
betas and other multi-factor betas, for estimating the volatility
(used for valuing options and for computing convexity effects), for
estimating default correlations in credit risk models and many other
Good classical statisticians are quite smart and in a practical
context would do many of the things discussed above when they have to
actually estimate a financial parameter. In my experience, they
usually agree that (a) there is a lot of randomness in historical
returns; (b) the data generating process does not remain unchanged for
too long; (c) therefore in practice there is not enough data to avoid
sampling error; and (d) hence it is desirable to use a method in which
sampling error is curtailed by fundamental judgement.
On the other side, Bayesians shamelessly use classical tools
because Bayes theorem is an omnivore that can digest any piece of
information whatever its source and put it to use to revise the prior
probabilities. In practical terms, Bayesians and classical
statisticians may end up doing very similar stuff.
The advantage of shifting to Bayesian statistics and subjective
probabilities is primarily conceptual and theoretical. It would
eliminate confusion in the minds of students on the ontological status
of the fundamental constructs of finance theory.
I am now therefore debating in my own mind whether finance
professors must spend some time in the class room discussing
How would it be like to begin the first course in finance with a
case study of subjective probabilities – something like the
delightful paper by Karl Borch (“The monster in Loch
Ness”, Journal of Risk and Insurance, 1976)? Borch
analyzes the probability that the Loch Ness
monster exists (and would be captured within a one year period)
given that a large company had to pay a rather high 0.25% premium to
obtain a million pound insurance cover from Lloyd’s of London
against that risk? This is obviously a question which a finance
student cannot refuse to answer; yet there is no obvious way to
interpret this probability in relative frequency terms.