Sunday, March 5, 2017

Python and R for code development

The previous post glossed about why I now prefer Python to write code, including for a module like logopt. This post explains in more details some specific differences where I prefer one of these two languages:
  • 0-based indexing in python versus 1-based indexing in R.  This may seem a small difference but for me, 0-based indexing is more natural and results in less off by one errors.  No less than Dijkstra opines with me on 0-based indexing.
  • = versus <- for assignment.  I like R approach here, and I would like to see more languages doing the same.  I still sometimes end up using = where I wanted ==.  If only R would allow <- in call arguments.
  • CRAN versus pypi
    • CRAN is much better for the user, the CRAN Task Views is a gold mine, and in general CRAN is a better repository, with higher quality packages.
    • But publishing one CRAN is simply daunting, and the reason logopt remained in R-Forge only.  The manual explaining how to write extensions is 178 pages long.
  • Python has better data structures, especially the Python dictionary is something I miss whenever I write in R.  Python has no native dataframe, but this is easily taken care of by importing pandas.
  • Object orientation is conceptually clean and almost easy to use in Python, less so in R.
  • Plotting is better in R.  There are some effort to make Python better in that area, especially for ease of use.  Matplotlib is powerful but difficult to master.
  • lm is a gem in R, the simplicity with which you can express the expressions you want to model is incredible
All in all, I prefer coding in Python.  This is a personal opinion of course, and R remains important because of some packages, but for more general purpose tasks, Python is simpler to use, and that translates in being more productive. 

Monday, February 20, 2017

Rebooting with Python and Jupyter

This blog has been inactive for a long time for essentially two reasons:

  • I was not very happy with the quality of the results
    • The source code was not showing very nicely
    • It was difficult to get a nice display, including for pictures and mathematical expressions
  • I started to use Python almost exclusively
    • R is a nice language, but it is not a general purpose language, some tasks are hard in R compared to Python
    • At the other hand, Python has steadily improved in the area of data processing, with pandas providing something equivalent to the R dataframe
But now, there is a good way to solve both problems, Jupyter notebooks combined with the ability to directly include HTML in a post.  So it is time for a reboot.

Jupyter was originally know as iPython but has evolved to support many programming languages, including R.  This allows now to develop a notebook, possibly based on multiple languages, then convert it for posting, while keeping the original notebook available for people that wants a more interactive experience.  The development process is much simpler that way that it used to be for earlier posts.

As an example, the rest of this post is this notebook converted to HTML.  Note that the notebook contains both R and Python code interacting in an almost seamless way.  How to achieve that result will be explained in later posts.

In [1]:
%load_ext rpy2.ipython
In [2]:
%%R -o x -o xik -o n -o pik
# figure 8.1 of Cover "Universal Portfolios"

library(logopt)
data(nyse.cover.1962.1984)
n <- nyse.cover.1962.1984
x <- coredata(nyse.cover.1962.1984)
xik <- x[,c("iroqu","kinar")]
nDays <- dim(xik)[1]
Days <- 1:nDays
pik <- apply(xik,2,cumprod)
plot(Days, pik[,"iroqu"], col="blue", type="l", 
     ylim=range(pik), main = '"iroqu" and "kinar"', ylab="")
lines(Days, pik[,"kinar"], col="red")
grid()
legend("topright",c('"iroqu"','"kinar"'),
       col=c("blue","red"),lty=c(1,1))
In [3]:
print(x)
print(type(x))
import matplotlib as mpl
import matplotlib.pyplot as plt
plt.ion()
plt.figure(figsize=(6,4))
plt.plot(pik)
plt.grid()
[[ 1.01515  1.02765  1.04183 ...,  1.00578  0.99697  0.99752]
 [ 1.01493  1.04036  0.98905 ...,  1.00958  0.99088  1.00248]
 [ 1.       0.97629  0.97786 ...,  1.       1.02761  0.99752]
 ..., 
 [ 0.99029  0.9966   0.99605 ...,  0.99216  1.00461  0.99273]
 [ 0.99265  1.00683  1.      ...,  0.99209  1.02752  1.00366]
 [ 0.99753  1.00339  1.01984 ...,  1.01195  1.       0.99635]]
<class 'numpy.ndarray'>

Thursday, November 22, 2012

Escaping the simplex, part 1

Before tackling the main subject, two quick notes:

  • I did not post for quite a while in part because I followed the Coursera online course Introduction to Computational Finance and Financial Econometrics.  It was a nice refresher, extremely well presented, and including some R.  This did consume enough time to make posting cumbersome though.
  • I started using R studio, and I am quite happy with it under Windows.  This has also impacted my code, as R studio automatically stacks your plots.
The main topic of this post is to motivate an escape from the simplex, i.e. allow for weights either larger than 1 or smaller than 0.  Generally, this implies going short on one asset deemed less favorable and shift the short proceed to a deemed more favorable asset.

The main reason to do so is that the Best Constant Rebalanced Portfolio (BCRP) may lie outside.  This is a well known feature in the more traditional mean-variance analysis, where the constrained portfolio is sparser than the unconstrained one.  We have a similar effect for the BCRP, the constrained optimization results in a sparse portfolio, i.e. many weights are (close to) zero.

We can illustrate this by calculating the weights of the constrained BCRP.  The BCRP function in logopt is currently only supporting optimization on the simplex, and we can reuse the code originally presented in Universal portfolio, part 10  to accumulate the portfolio weights for BCRP across multiple combinations of the reference data.  This shows that the proportion of (close to) zero weights is high.

The code below as is runs for about 7 hours because of the large number of combinations, you can reduce the size of the problem if wanted by editing TupleSizes.

Note that in the past Syntax Highlighter had problems with R-bloggers, you might need to go to the original page to view the code.

# assume the use of RStudio (no explicit management of graphic devices)

library(logopt)
data(nyse.cover.1962.1984)
x <- coredata(nyse.cover.1962.1984)
nStocks <- dim(x)[2] 

EvaluateOnAllTuples <- function(ListName, TupleSizes, fFinalWealth, ...) {
  if (exists(ListName) == FALSE) {
    LocalList <- list()
    for (i in 1:length(TupleSizes)) {
      TupleSize <- TupleSizes[i]
      ws <- combn(x=(1:nStocks), m=(TupleSize), FUN=fFinalWealth, simplify=TRUE, ...)
      LocalList[[i]] <- ws
    }
    assign(ListName, LocalList, pos=parent.frame())
  }
}

TupleSizes <- c(2,3,4,5)

# evaluate the sorted coefficients for best CRP
SortedOptB <- function(cols, ...) {
  x <- list(...)[[1]] ;
  x <- x[,cols]
  b <- bcrp.optim(x)
  return(sort(b))
}

TupleSizes <- c(2,3,4,5)
EvaluateOnAllTuples("lSortedOptBSmall", TupleSizes, SortedOptB, x)

TupleSizes <- c(nStocks-3,nStocks-2,nStocks-1,nStocks)
EvaluateOnAllTuples("lSortedOptBLarge", TupleSizes, SortedOptB, x)

Colors <- c("red", "green", "blue", "brown", "cyan", "darkred","darkgreen")

for (iL in 1:length(lSortedOptBSmall)) {
  nCoeff <- nrow(lSortedOptBSmall[[iL]])
  E <- ecdf(lSortedOptBSmall[[iL]][1,])
  Title <- sprintf("Cumulative PDF for all sorted weights for BCRP of %d assets", nCoeff)
  plot(E, col=Colors[1], xlim=c(0,1), pch=".", main = Title, xlab = "relative weight")
  cat(sprintf("Combinations of %d assets\n", nCoeff))
  for (iB in 1:nCoeff) {
    E <- ecdf(lSortedOptBSmall[[iL]][iB,])
    lines(E, col=Colors[iB], pch=".")
    if (iB < nCoeff) {
       cat(sprintf("Percent of portfolio with %d weight(s) smaller than 0.001: %f\n", iB, E(0.001) ))
    }
  }
}

SmallOf2 <- lSortedOptBSmall[[1]][1,]
hist(SmallOf2,n=25,probability=TRUE, main="Histogram of smallest weight for two assets", 
     xlab="Smallest coefficient")
lines(density(SmallOf2, bw=0.02),col="blue")

# for the large ones, we show only the largest coefficients and show them in 
# opposite order to find how many coefficients are not always insiginifcant
for (iL in 1:length(lSortedOptBLarge)) {
  nCoeff <- nrow(lSortedOptBLarge[[iL]])
  E <- ecdf(lSortedOptBLarge[[iL]][nCoeff,])
  Title <- sprintf("Cumulative PDF for largest sorted weights for BCRP of %d assets", nCoeff)
  plot(E, col=Colors[1], xlim=c(0,1), pch=".", main = Title, xlab = "relative weight")
  cat(sprintf("Combinations of %d assets\n", nCoeff))
  for (iB in 1:nCoeff) {
    iCoeff <- nCoeff-iB+1
    E <- ecdf(lSortedOptBLarge[[iL]][iCoeff,])
    if (E(0.001) < 0.9999) {
      lines(E, col=Colors[iB], pch=".")
      cat(sprintf("Percent of portfolio with %d weight(s) smaller than 0.001: %f\n", iCoeff, E(0.001) ))
    }
  }
}

Only a subset of the results are shown below, first the cumulative probability function for the weights of the coefficients for all combinations of 5 assets.  The graph shows that the smallest coefficient is almost always 0 and the second coefficient is also very small all of the time.  The textual output shows that the second coefficient is insignificant 91% of the time or equivalently 91% of the best portfolio only uses 3 of the 5 possible assets.


Combinations of 5 assets
Percent of portfolio with 1 weight(s) smaller than 0.001: 0.996499
Percent of portfolio with 2 weight(s) smaller than 0.001: 0.914086
Percent of portfolio with 3 weight(s) smaller than 0.001: 0.499448
Percent of portfolio with 4 weight(s) smaller than 0.001: 0.067975



The density for the two asset case shows clearly that there is a high peak at zero.


Finally the textual output and the ECDF shows that for 33 possible assets, only up to 7 are present in the BCRP


Combinations of 33 assets
Percent of portfolio with 33 weight(s) smaller than 0.001: 0.000000
Percent of portfolio with 32 weight(s) smaller than 0.001: 0.000000
Percent of portfolio with 31 weight(s) smaller than 0.001: 0.000000
Percent of portfolio with 30 weight(s) smaller than 0.001: 0.000000
Percent of portfolio with 29 weight(s) smaller than 0.001: 0.065126
Percent of portfolio with 28 weight(s) smaller than 0.001: 0.707843
Percent of portfolio with 27 weight(s) smaller than 0.001: 0.947059


All this points to the fact that most of the weights of the BCRP end up on the boundary of the simplex, and that removing that specific constraint would get an even better solution, at least in term of terminal wealth.  We'll investigate further this in future posts.

Sunday, September 23, 2012

Universal portfolio, part 11

First an apology, the links to the Universal Portfolio paper have stopped working.  This is because the personal webpage of Thomas Cover at Stanford has been taken down, but fortunately the content moved elsewhere.  The new link is Universal Portfolio and hopefully this one will be stable.

Note that there are many available copies on the web but most (like this one) are for something that seems to be a slighly reworked version dated October 23 1996.  The text appears mostly identical to the published version, but it does not include the figures.

In the rest of this post, I discuss the data used by Cover.  That data is included in logopt as nyse.cover.1962.1984.  It contains the relative prices for 36 NYSE stocks between 1962 and 1984.


> range(index(nyse.cover.1962.1984))
[1] "1962-07-03" "1984-12-31"
> colnames(nyse.cover.1962.1984)
 [1] "ahp"    "alcoa"  "amerb"  "arco"   "coke"   "comme"  "dow"    "dupont" "espey"  "exxon"  "fisch"  "ford"   "ge"     "gm"     "gte"    "gulf"   "hp"    
[18] "ibm"    "inger"  "iroqu"  "jnj"    "kimbc"  "kinar"  "kodak"  "luken"  "meico"  "merck"  "mmm"    "mobil"  "morris" "pandg"  "pills"  "schlum" "sears" 
[35] "sherw"  "tex" 

The names are not stickers, some guessing and with some help from an other person using the series gives the table below (and if anybody knows about the one without expansion yet. please post a comment).

Abbreviation Company name Current ticker
ahp ? ?
alcoa Alcoa AA
amerb American Brands
aka Fortune Brands
-
arco ? ?
coke Coca-Cola KO
comme Commercial Metals CMC
dow Dow Chemicals DOW
dupont DuPont DD
espey Espey Manufacturing ESP
exxon Exxon Mobil XOM
coke Coca-Cola KO
fisch Fischbach Corp -
ford Ford F
ge General Electric GE
gm General Motors GM*
gte GTE Corporation -
gulf Gulf Oil (now Chevron) CVX
hp Hewlett-Packard HPQ
ibm IBM IBM
inger Ingersoll-Rand IR
iroq Iroquois Brands -
jnj Johnson & Johnson JNJ
kimbc Kimberly-Clark KMB
kinar Kinark? -
kodak Eastman Kodak EKDKQ
luken Lukens? -
meico ? ?
merck Merck MRK
mmm 3M MMM
mobil Exxon Mobil XOM
morris Philip Morris PM
pandg Procter & Gamble PG
pills Pillsbury, now part of General Mills -
schlum Schlumberger SLB
sears Sears Holdings SHLD
sherw Sherwin-Williams SHW
tex Texaco, now Chevron CVX

There is a lot of diversity across the different stocks, we saw that in two ways:

  • by showing the global time evolution of all stocks in time
  • by showing the growth rate at two times separated by N market days (shown as a price relative between the two dates).

# Some statistics on the NYSE series

library(logopt)
x <- coredata(nyse.cover.1962.1984)
w <- logopt:::x2w(x)
nDays <- dim(x)[1]
nStocks <- dim(x)[2] 
Days <- 1:nDays
iWin <- 1 ; plot(1:10)
Time <- index(nyse.cover.1962.1984)

# for each stock calculate:
# - min, max
# - average geometric return

MaxFinal <- max(w[nDays,])
MinFinal <- min(w[nDays,])
MaxAll <- max(w)
MinAll <- min(w)

if(length(dev.list()) < iWin) { x11() } ; iWin <- iWin + 1 ; dev.set(iWin) ;
plot(Time, w[,1], col="gray", ylim=range(w), log="y", type="l")
for (i in 1:nStocks) {
 lines(Time, w[,i], col="gray")
 if (w[nDays,i] == MaxFinal) { cat(sprintf("Stock with best final value: %s finishing at %.2f\n", colnames(w)[i], MaxFinal)) ; iMax <- i }
 if (w[nDays,i] == MinFinal) { cat(sprintf("Stock with worst final value: %s finishing at %.2f\n", colnames(w)[i], MinFinal)) ; iMin <- i }
 if (max(w[,i]) == MaxAll) { cat(sprintf("Stock with best peak value: %s at %.2f\n", colnames(w)[i], MaxAll)) }
 if (min(w[,i]) == MinAll) { cat(sprintf("Stock with worst valley value: %s at %.2f\n", colnames(w)[i], MinAll)) } 
}

lines(Time, w[,iMax], col="green")
lines(Time, w[,iMin], col="red")
lines(Time, apply(w,1,mean), col="blue")
grid()

# do a summary across n quotes
nDelta <- 1200
wD <- w[(nDelta+1):nDays,] / w[1:(nDays-nDelta),]
Time <- Time[1:(nDays-nDelta)]
MaxDAll <- max(wD)
MinDAll <- min(wD)
if(length(dev.list()) < iWin) { x11() } ; iWin <- iWin + 1 ; dev.set(iWin) ;
plot(Time, wD[,1], col="gray", ylim=range(wD), log ="y", type="l")
for (i in 1:nStocks) {
 lines(Time, wD[,i], col="gray")
 if (max(wD[,i]) == MaxDAll) { cat(sprintf("Stock with best gain on %s days: %s at %.2f\n", nDelta, colnames(w)[i], MaxDAll)) }
 if (min(wD[,i]) == MinDAll) { cat(sprintf("Stock with worst lost on %s days: %s at %.2f\n", nDelta, colnames(w)[i], MinDAll)) } 
} 
lines(Time, apply(wD,1,mean), col="blue")

grid()

This gives the following textual answer and graphs.  Note that there are many alternate ways to present this information, in particular the package PerformanceAnalytics.


Stock with worst final value: dupont finishing at 3.07
Stock with worst valley value: meico at 0.26
Stock with best final value: morris finishing at 54.14
Stock with best peak value: schlum at 90.12
Stock with best gain on 1200 days: espey at 15.84
Stock with worst lost on 1200 days: meico at 0.07



This sequence forms a nice reference covering a long period of time, and has been used in many studies of portfolio selection algorithms.  But the series has a number of serious problems:
  • Survivorship bias
  • The time range corresponds to a time where quotes were not yet decimal.

Friday, August 10, 2012

Universal portfolio, part 10

Part 9 compared the wealth of Universal against other portfolio selection algorithms by using the experimental cumulative distribution function of the relative wealth.  This leads to a very compact representation, but it completely hides the absolute level evolution as the number of stocks in a portfolio increases.

The code below uses a slightly different approach, it uses a scatterplot where the final absolute wealth of two different algorithms are used for the x and y axes.  The main diagonal corresponds to both algorithms having an equal final wealth.  To provide more information, side graphs with the marginal probability of final wealth are included.

Finally, an other reference is added, the best CRP, using the optimization code in package logopt.

As in part 9, the code recalculates the value of Universal final wealth across all 4-tuples and thus takes one day to run if you did not have yet the results in your environment, be warned. Also, the code presentation uses Syntax Highlighter, I am still experimenting with the best way to present R code.


# Performance of Universal compared to some references

library(logopt)
x <- coredata(nyse.cover.1962.1984)
w <- logopt:::x2w(x)
nDays <- dim(x)[1]
nStocks <- dim(x)[2] 
Days <- 1:nDays
iWin <- 1 ; plot(1:10)

TupleSizes <- c(2,3,4)
EvaluateOnAllTuples <- function(ListName, TupleSizes, fFinalWealth, ...) {
 if (exists(ListName) == FALSE) {
  LocalList <- list()
  for (i in 1:length(TupleSizes)) {
   TupleSize <- TupleSizes[i]
   ws <- combn(x=(1:nStocks), m=(TupleSize), FUN=fFinalWealth, simplify=TRUE, ...)
   LocalList[[i]] <- ws
  }
  assign(ListName, LocalList, pos=parent.frame())
 }
}

UniversalFinalWealth <- function(cols, ...) {
 x <- list(...)[[1]] ; n <- list(...)[[2]]
 uc <- universal.cover(x[,cols], 20)
 return(uc[length(uc)])
}
EvaluateOnAllTuples("lUniversalFinalWealth", TupleSizes, UniversalFinalWealth, x, 20)

BestStockFinalWealth <- function(cols, ...) { 
 w <- list(...)[[1]]
 return(max(w[nDays,cols])) 
}
EvaluateOnAllTuples("lBestStockFinalWealth", TupleSizes, BestStockFinalWealth, w)

UcrpFinalWealth <- function(cols, ...) {
 x <- list(...)[[1]]
 ucrp <- crp(x[,cols])
 return(ucrp[length(ucrp)]) 
}
EvaluateOnAllTuples("lUcrpFinalWealth", TupleSizes, UcrpFinalWealth, x)

BhFinalWealth <- function(cols, ...) {
 x <- list(...)[[1]]
 ubh <- bh(x[,cols])
 return(ubh[length(ubh)]) 
}
EvaluateOnAllTuples("lBhFinalWealth", TupleSizes, BhFinalWealth, x)

BestCrpFinalWealth <- function(cols, ...) { 
 x <- list(...)[[1]]
 bopt <- bcrp.optim(x[,cols])
 bcrp <- crp(x[,cols],bopt)
 return(bcrp[length(bcrp)]) 
}
EvaluateOnAllTuples("lBestCrpFinalWealth", TupleSizes, BestCrpFinalWealth, x)

Colors <- c("blue","green","red")

CompareFinalAbsoluteWealth <- function( L0, L1, MainString, TupleSizes=TupleSizes, clip=0.01,
                           Colors=c("blue","green","red"), PlotChar = 19, PlotSize = 0.5,
                                   XLabel="Universal wealth", YLabel="Other wealth") {
 nLines <- min(length(L0),length(L1))
 if (clip > 0) { MaxUp <- quantile(c(L0[[nLines]], L1[[nLines]]), 1-clip) }
 else          { MaxUp <- max(L0, L1) }
 XLims = c(0, MaxUp)
 layout( matrix( c(0,2,2,1,3,3,1,3,3),ncol=3) )
 d.x <- density(L0[[1]])
 plot(d.x$x, d.x$y, xlim=XLims, type='l', col=Colors[1], main="Density on x axis", xlab="", ylab="")
 grid()
 for (i in 1:nLines) {
  d.x <- density(L0[[i]])
  lines(d.x$x, d.x$y, type='l', col=Colors[i])
  abline(v=mean(L0[[i]]), col=Colors[i])
 }
 d.y <- density(L1[[1]])
 plot(d.y$y, d.y$x, ylim=XLims, xlim=rev(range(d.y$y)), type='l', col=Colors[1], , main="Density on y axis", xlab="", ylab="")
 grid()
 for (i in 1:nLines) {
  d.y <- density(L1[[i]])
  lines(d.y$y, d.y$x, type='l', col=Colors[i])
  abline(h=mean(L1[[i]]), col=Colors[i])
 }
 plot(L0[[nLines]], L1[[nLines]], col=Colors[nLines], pch=PlotChar, cex=PlotSize, 
  xlab=XLabel, ylab=YLabel, xlim= XLims, ylim= XLims, type="p", main=MainString) 
 for (j in 1:nLines) {
  i <- nLines - j + 1
  points(L0[[i]], L1[[i]], col=Colors[i], pch=PlotChar, cex=PlotSize) 
  rug(L0[[i]], col=Colors[i],ticksize=0.01*i)
  rug(L1[[i]], col=Colors[i],ticksize=0.01*i, side=2)
 }
 abline(0,1,col="lightgray",lwd=2)
 legend("topright", legend=c("2 stocks","3 stocks","4 stocks"), pch=PlotChar, col=Colors, bg="white")
 grid()
}


if(length(dev.list()) < iWin) { x11() } ; iWin <- iWin + 1 ; dev.set(iWin) ;
CompareFinalAbsoluteWealth(lUniversalFinalWealth, lBestStockFinalWealth, "Universal relative to best stock final wealth", TupleSizes) 

if(length(dev.list()) < iWin) { x11() } ; iWin <- iWin + 1 ; dev.set(iWin) ;
CompareFinalAbsoluteWealth(lUniversalFinalWealth, lBhFinalWealth, "Universal relative to uniform buy and hold final wealth", TupleSizes) 

if(length(dev.list()) < iWin) { x11() } ; iWin <- iWin + 1 ; dev.set(iWin) ;
CompareFinalAbsoluteWealth(lUniversalFinalWealth, lUcrpFinalWealth, "Universal relative to uniform CRP final wealth", TupleSizes) 

if(length(dev.list()) < iWin) { x11() } ; iWin <- iWin + 1 ; dev.set(iWin) ;
CompareFinalAbsoluteWealth(lUniversalFinalWealth, lBestCrpFinalWealth, "Universal relative to best CRP final wealth", TupleSizes) 

# a function to compare the ECDF of two lists of final wealths
CompareFinalRelativeWealth <- function( L0, L1, MainString, TupleSizes=TupleSizes, 
                              Colors=c("blue","green","red"), PlotChar = ".",
                                   XLabel="Ratio of final wealths", YLabel="Cumulative probability") {
 nLines <- min(length(L0),length(L1))
 LR <- list() ; XLims = c()
 for(i in 1:nLines) { LR[[i]] <- L0[[i]]/L1[[i]] ; XLims <- range(XLims, LR[[i]]) }
 plot(ecdf(L0[[1]]/L1[[1]]), pch=PlotChar, col=Colors[1], main=MainString,
  xlab=XLabel, ylab=YLabel, xlim= XLims)
 abline(v=1,col="gray",lwd=2)
 for (i in 1:nLines) {
  lines(ecdf(L0[[i]]/L1[[i]]), pch=PlotChar, col=Colors[i])
 }
 legend("bottomright", legend=c("2 stocks","3 stocks","4 stocks"), fill=Colors)
 grid()
 # show best relative wealth and its composition
 for (i in 1:length(TupleSizes)) {
  TupleSize <- TupleSizes[i]
  BestTuple <- which.max(LR[[i]])
  BestStocks <- combn(1:nStocks, TupleSize)[,BestTuple]
  cat(sprintf("Max final relative wealth %.4f for stocks: ", LR[[i]][BestTuple]))
  cat(colnames(x)[BestStocks]) ; cat("\n")
 }
}

if(length(dev.list()) < iWin) { x11() } ; iWin <- iWin + 1 ; dev.set(iWin) ;
CompareFinalRelativeWealth(lUniversalFinalWealth, lBestCrpFinalWealth, "Universal relative to best CRP final wealth", TupleSizes) 

The first graph compares Universal to the best stock in the portfolio.  It also shows that the density function is not an ideal solution when the set of possible outcomes is discrete.  The ecdf function would be a better choice in this case.  The plot is also slightly misleading when compared to the equivalent plot of relative wealth in part 9.  The problem is that points overlap and so the 2D density cannot be assessed except for the set of blue points.


The next graph shows a comparison of Universal and Uniform CRP.  The graph works reasonably well in this case because both axes have smooth marginal distributions.  As for the corresponding graph of relative wealth, we can clearly see that UCRP is simply better, but now we can also see that this is especially true for the best performing portfolios.

The comparison between Universal and Uniform Buy and Hold below also works well.  And this time also we can see that the ratio gets better for better performing portfolios, but now with Universal the better algorithm


Finally the comparison between the Best CRP (in hindsight) and Universal shows that BCRP is always better.  Because the probability density function of the best CRP is less smooth, the comparison is not perfect, but it seems that the ratio degrades as the number of stocks in the portfolio increases.  This is expected given that the performance bound of Universal decreases as the number of stocks increase.


Plotting the ECDF of the relative wealth shows this effect much more vividly, clearly illustrating the advantage of looking at the same data in different fashions.



Wednesday, July 25, 2012

Universal portfolio, part 9

Part 8 was discussing the distribution of the absolute wealth of the Universal Portfolio across all possible tuples of length 2, 3 and 4.

However, comparing the absolute wealth against some reference, especially against simple portfolio selection algorithm provides a better view of the exact performance of the Universal algorithm.  Because we want to compute and compute multiple references, the code uses functions to be more generic.  The code remains terse (88 lines, including comments) and could be even terser.

Writing the code in this fashion shows again the strength of R, some specific aspects used here:

  • Passing functions as parameters of other functions
  • Using frames to access information outside the function scope, including assigning new values.
  • Use of ... to write variadic functions (I am still learning that, the current code works but is not elegant)
  • Use of ::: to access non exported functions from a package
The code recalculates the value of Universal final wealth across all 4-tuples and thus takes one day to run, be warned.






The first graph shows the final wealth of Universal versus the final wealth of the best stock in the tuple. Contrary to the absolute wealth, the relative wealth decreases as the number of stocks in the tuple increases and is also generally significantly less than 1.  The red curve shows that for about 70% of the 4-tuples, Universal final wealth is below the wealth of the best stock in the tuple.


Obviously, it is impossible to know in advance which stock will have the best final wealth, so the comparison is slightly unfair.  The next two comparisons however are against two of the simplest causal selection algorithms:

  • Equally weighted buy and hold, UBH for short, U for uniform
  • Equally weighted Constant Rebalanced Portfolio, usually UCRP for Uniform CRP.  This one is interesting as Universal itself is based on a weighted combination of all CRP.
As BH is by construction worse than the best stock, we expect BH to fare better and indeed it does.  First Universal is generally better than BH, and second the relative performance increases as the number  of stocks in the tuple increase.  In general, Universal is much better than UBH.  The cumulative distribution does still shows a rather long and thin tail.


Unfortunately this does not carry to UCRP, UCRP happens to be a very good performer, and a very tough reference to beat for any portfolio selection algorithm.


Against UCRP:
  • Universal generally loses to UCRP (around 90% of the time) and sometimes pretty badly
  • Increasing the tuple size increases the relative performance when UCRP is better than Universal and decreases the performance when the opposite is true.
Interestingly, the textual output shows that the composition of the best absolute or relative Universal portfolios differ significantly for the comparison against UCRP.

Max final wealth 78.4742 for stocks: comme kinar
Max final wealth 111.6039 for stocks: comme kinar meico
Max final wealth 138.5075 for stocks: comme espey kinar meico
Max final relative wealth 4.3379 for stocks: iroqu kinar
Max final relative wealth 5.6186 for stocks: espey iroqu kinar
Max final relative wealth 5.2758 for stocks: coke espey iroqu kinar
Max final relative wealth 5.9302 for stocks: iroqu kinar
Max final relative wealth 8.6374 for stocks: espey iroqu kinar
Max final relative wealth 8.8390 for stocks: espey iroqu kinar meico
Max final relative wealth 1.3043 for stocks: dupont morris
Max final relative wealth 1.1980 for stocks: dupont morris sears
Max final relative wealth 1.1330 for stocks: dupont morris schlum sears