We consider the problem of sequentially investing in a set of stocks.
However to start with we begin with the example of gambling on horses.
- Consider a sequence of horse races.
- Each race has horses and only one hose can win.
- Let be the unit vector indicating the winning horse in the th race i.e. with the one in the position of the th winning horse. Further let
- Let be the proportion of our wealth that we invest in horse given the winners so far .
- If horse wins we multiply our wealth by and for all .
- We wish to compare the performance of our prediction policy for betting and some set of experts .
We want to compare the performance of our policy with the best expert. First we show that we can express this in a regret framework.
Ex 1. Argue that after races, the ratio between our wealth and the best expert in is given by
and similarly for .
Ans 1. Starting with an initial wealth of , the wealth attained from following and respectively are
Now divide the second term by the first and maximize over .
Ex 2. [Continued] Argue that the above ratio can be expressed in terms of regret:
where the loss of a probability distribution and horse winning is given by
Ans 2. Take logs in  and
Def 3. [Minimax Regret] We look at the worse regret that a specific strategy can have relative to experts ,
and then we look for the best strategy
Here is called the minimax regret.
As we will see there is a unique policy that achieves this regret.
Def 4. [Normalized Maximum Likelihood Forcaster] This is the policy defined by
You can bet a proportion wealth
where there is only a prize for winning
Ex 5. For any class of experts and integrer , the normalized maximum likelihood forcaster is the unique forecaster such that
Ans 5. For a distribution such that (remembering both distributions sum to one). So,
Ex 6. [Continued] Show that for all
Ans 6. This follows from the answer to .
We now develop put theory for investment strategies. Here there is now longer one winning horse/stock. We consider the goal of maximizing final wealth with respect to a set of reference strategies.
- An investor trades over days.
- We consider a market with stocks.
- A market vector gives the (relative) prices of stocks over each period given period.
- For a sequence of market vectors , , we let denote the history of market vectors before time .
- An investment is a probability distribution that give the proportions of (one unit of) wealth invested in each asset. This results in an increase in wealth of
- An investment strategy, , is a sequence of investments for each time .
Def 7. [Wealth Factor] The wealth factor of an on investment strategy is given by
Def 8. [Market Vector] For a vector giving the relative prices of stocks, a Market vector investment in each of these stocks and this results in an increase in wealth
Def 9. [Wealth Factor] For a sequence of investments and stock prices and , the wealth factor is the increase in wealth from time
Def 10. [Investment Strategy] An investment strategy is a sequence of functions , each giving a market vector.
Ex 11. [Buy-and-Hold] Buy-and-hold, , is an investment strategy where you distribute you initial wealth according to some fixed proportion , and afterwards you don’t trade. Show that here the wealth factor is
Ans 11. It is clear the increase in asset is so the average increase is as given.
Ex 12. [Constantly rebalanced portfolio] A constantly rebalanced portfolio, , is a policy that keeps a fixed proportion of wealth, , invested in each asset. Show that here the wealth factor is
Ans 12. Now the increase in wealth at time is
Thus the total increase is the product of these terms.
Def 13. [Minimax Regret] For a class of investment strategies , the regret of an investment strategy is (similar to )
The minimax regret is then
Ex 14. [Interpreting the minimax wealth ratio] Show that if then has the same growth rate as the best investment strategy in , i.e. for all
(From this exercise we can interpret the regret as the factor from which we are away from the best expert.)
Ex 15. Show that if
then there exists a policy such that for all histories
(i.e. The wealth factor of the policu is guarenteed to be within a factor of the best policy in .)
The minimax wealth ratio is the best possible logarithmic wealth ratio
Def 16. [Kelly Market Vector] A market vector that has only one non-zero component is called a Kelly Market vector, i.e. .
Note that we considered exactly Kelly market vectors in the previous horse race example.
We want to consider the differences for the “investment policies“ and “prediction policies” for the (horse) betting problem considered previously.
For every investment policy we can define a prediction policy:
Ex 17. Given an investment policy we can define a prediction policy induced by by
Similarly, for every prediction policy we can define an investment policy:
Ans 17. Given a prediction strategy (as considered in the horse racing example), we can define an investment strategy induced by by
Nb, note that the normalization constant is given by
Ex 18. The wealth factor the investment strategy induced prediction policy is
where is given as above in Def. [SI:PinduceI].
Ans 18. From Def. 7, . Looking at each term in this product
In prinicple the investment problem should be more difficult that the prediction problem (used in horse racing). But the result below shows that actually they are in some sense equivalent.
Thrm 19. For a set of constantly rebalanced portfolios and for the class of Prediction strategies induced by (cf. Def 17), we have that
Moreover, in this case, the minimax optimal investment strategy is the investment strategy induced by the normalized maximum likelihood prediction policy.
The first example confirms the intuition that investing is harder than prediction:
Ex 20. [Continued – from Thrm 19] Show that
The first inequality holds since the Kelly market vectors are a subset of the set of Market Vectors. The subsequent equality holds since
and we can replace policies in and since then coincide on Kelly market vectors (cf. Def 17). The final equality and inequality follow by the definition of the minimax regret in the horse racing example (Def. 3).
Ex 21. Show that for non-negative numbers and
Ex 22. Show that the wealth factor for any investment policy can be expressed as a sum over the Kelly market vectors
Ex 23. Show that for the investment strategy induced by the normalized maximum likilihood policy, we have that
(Def. ?? = Def. 13, [??,??]= [18,22], [??]=.)
Ex 24. [Continued] Show that for the set of constantly rebalanced portfolios and for the normalized maximum likilihood policy for , we have that
Ans 24. For a constantly rebalanced portfolio, .
Here [??] =  , Def. ?? = Def. 4, and [??] = .
Ex 25. [Continued] Show that Thrm 19 holds. That is that
Ans 25.(Here [??]=) and by  the inequalities also hold in the other direction.