fitting copulas: some tips andreas tsanakas, cass business school staple inn, 16/11/06
TRANSCRIPT
Tip #1: Look at your data (1)
Usually there are some data, even if they are not enough to run a formal Maximum Likelihood Estimation process
Plot what you have and look at some heuristics They may help you decide with model choice and sensible parameter
ranges
Example Plot ranks Visually test for tail-dependence Visually test for skewness
Tip #1: Look at your data (2)
Sample ranks are a data set on which a copula may be fitted So work with those
There are dependence measures that relate only to these ranks Spearman’s rank correlation Kendall tau Blomqvist beta
The estimates of these tend to be more stable than of the usual correlation
Can use directly to parameterise some models Kendall’s tau works well with elliptical (Gaussian, t) and Archimedean
(Gumbel, Clayton) copulas
Rank plots
Dependence patterns do not get distorted by marginals
‘Real world’ ‘Copula world’
R2 = 0.05
0
0.5
1
1.5
2
2.5
3
3.5
4
0 500 1000 1500 2000 2500
X1
X2
R2 = 0.64
0
0.25
0.5
0.75
1
0 0.25 0.5 0.75 1
U1
U2
Tail correlations
A local dependence metric that is sensitive to asymptotic tail dependence (based on Schmidt and Schmid, 2006)
Left tail p=0
Right tail p=1
Compare 3 copulas
- Gaussian
- t
- Asymmetric t Plot using 20,000 Samples
Very unstable!
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Confidence level
Ta
il-B
lom
qvi
st c
orr
ela
tion
Gaussian T (dof=3)Asym. T
2
2
pp
pp,pCpβ
Skew-rank correlations
Try to identify skewness from a small sample (of 20) Generalisation of rank correlation with a 3rd moment adjustment
The sensitivity to the 3rd joint moments of the
sample ranks increases with coefficient “a”.
a=0 gives the usual rank correlation.
0 2 4 6 8 10 12 14 16 18 20-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Coefficient a
Ske
w R
an
k C
orr
ela
tion
Gaussian T (dof=3)Asym. T
Quadrant correlations Can write (rank) correlation as sum of correlations in each quadrant
(Smith, 2002)
Gaussian copula
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Rank(X)
Ra
nk(
Y)
Quadrant correlations Can write (rank) correlation as sum of correlations in each quadrant
(Smith, 2002)
Gaussian copula
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Rank(X)
Ra
nk(
Y)
Q1
Q4
Q3
Q2
Quadrant correlations Can write (rank) correlation as sum of correlations in each quadrant
(Smith, 2002)
t copula
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Rank(X)
Ra
nk(
Y)
Quadrant correlations Can write (rank) correlation as sum of correlations in each quadrant
(Smith, 2002)
Asymmetric t copula
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Rank(X)
Ra
nk(
Y)
Quadrant correlations Plot the % breakdown of aggregate rank correlation to quadrants
0 1 2 3 4 5-4
-3
-2
-1
0
1
2
3
4
5
Quadrant
% B
rea
kdo
wn
of c
orr
ela
tion
to q
ua
dra
nts
Gaussian T (dof=3)Asym. T
Tip #2: Use judgement expertly
There are 3 ways of using expert judgement in model calibration Method 1
Ask underwriters and other experts for sources of correlation Make up some numbers
Method 2 Ask questions that experts can answer meaningfully Identify drivers of dependence Associate those with your copula models
Method 3 Adopt a full formal Bayesian framework Not quite there yet!
Tip #3: Understand your models
Copula models can often be expressed via factors (drivers) So not as ad-hoc as you may think Helps you chose an appropriate structure
Gaussian copula: n (or less!) additive factors determine correlation structure
t-copula: same as Gaussian with one additional multiplicative factor driving tail dependence, even for otherwise uncorrelated risks
Archimedean copulas (e.g. Gumbel, Clayton): there is one factor, conditional upon which all risks are independent
p-factor Archimedean copulas: as above, but can use more than one factor
Tip #4: Keep it simple
Modern DFA software offers you a wealth possibilities But you may be tempted to overparameterise your model A small number of parameters (drivers) will give you a better
chance of making meaningful (interpretable) choices You may even be able to do a bit of estimation
Tip #5: Use other models for reference
Suppose you have a peril model which you believe in But it doesn’t cover all your risks
Maybe the dependence structure that it implies between classes can be used as a proxy for risks that aren’t covered?
Fit a copula to the peril model simulations Loads of (pseudo-)data!
Alternatively suppose you have some particular pairs of risks for which you have more data This may help you get a feeling for sensible parameter ranges
Tip #6: Work backwards
See what diversification credits different copula choices imply Do they make sense? The argument is of course circular, but it may help involve people
in the thinking
Tip #7: Take care of your data
You may not have enough data today If you collect and maintain your data you’ll have more tomorrow
Tip #8: Adopt a positive attitude
It is no good complaining about fitting a copula ‘being impossible’ It is difficult but not impossible in principle Insisting on the difficulty does not make the problem of dependencies
go away
There are some things you can do Though they still may not work Shouldn’t you be used to this?
Literature I used for this talk
On most things: McNeil, Frey and Embrechts (2005), Quantitative Risk Management,
Princeton University Press.
On quadrant correlations and normal mixtures Smith (2002), ‘Dependent Tails’, 2002 GIRO Convention,
http://www.actuaries.org.uk/Display_Page.cgi?url=/giro2002/index.xml
A technical paper from which I pinched an idea or two: Schmidt and Schmid (2006) ‘Nonparametric Inference on Multivariate Versions
of Blomqvist's Beta and Related Measures of Tail Dependence’, http://www.uni-koeln.de/wiso-fak/wisostatsem/autoren/schmidt/publication.html
Sample literature on fitting copulas (yes it exists)
Genest and Rivest (1993), ‘Statistical inference procedures for bivariate Archimedean copulas,’ J. Amer. Statist. Assoc. 88, 1034-1043.
Joe (1997), Multivariate Models and Dependence Concepts, Chapman & Hall.
McNeil, Frey and Embrechts (2005), Quantitative Risk Management, Princeton University Press.
Denuit, Purcaru and Van Bellegem (2006), 'Bivariate archimedean copula modelling for censored data in nonlife insurance'. Journal of Actuarial Practice 13, 5-32.
Chen, Fan and Tsyrennikov (2006), ‘Efficient estimation of semiparametric multivariate copula models. J. Am. Stat. Assoc. 101, 1228-1240.
Appendix - dependence measures
Consider risks X and Y, with cdfs F and G. Let U=F(X), V=G(Y) Let X’=X-E[X], Y’=Y-E[Y], U’=U-E[U], V’=V-E[V] Assume sample of size n x={x1,…,xn } is the sample from random variable X etc
u={u1,…,un } are the normalised sample ranks of X, i.e. numbers 1/n, 2/n,…,1, but ordered in the same way as the elements of x.
Same for y, v.
Appendix - dependence measures
Pearson correlation coefficient
Spearman correlation coefficient
Spearman correlation for the Gaussian copula
where r is the Pearson correlation of the underlying normal distribution
22 'YE'XE
'Y'XEY,Xρ
V,UρY,XρS
Y,Xρr2
1arcsin
π
6Y,XρS
Appendix - dependence measures
Kendall correlation coefficient (population version)
where is an independent copy of (X,Y) Kendall correlation coefficient (sample version)
Kendall correlation coefficient elliptical copulas (incl. Gaussian, t)
where r is the Pearson correlation of the underlying elliptical distribution
0Y~
YX~
XP0Y~
YX~
XPY,Xτ
)Y~
,X~
(
nji1ijij
1
yyxxsign2
nY,Xτ
rarcsinπ
2Y,Xτ
Appendix - dependence measures
Kendall correlation coefficient of the Pareto (flipped Clayton) copula
Copula function:
Kendall’s τ:
Kendall correlation coefficient of the Gumbel copula
Copula function:
Kendall’s τ:
θ/1θθ 1v1u11vuv,uC
θ/1θθ vlnulnexpv,uC
2θ
θ
θ
11
Appendix - dependence measures
“Tail-Blomqvist” correlation coefficient (Schmidt and Schmid, 2006)
where 0<p<1 and C is the copula of (X, Y) For we have asymptotic upper tail-dependence
For we have asymptotic lower tail-dependence
For sample version use the empirical copula:
2
2
pp
pp,pCp;Y,Xβ
0λp;Y,Xβlim U1p
0λp;Y,Xβlim L0p
2
2
pp
pp,pCp;Y,Xβ
n
n/jv,n/iu:)v,u(#
n
j,
n
iC kkkk
n
Appendix - dependence measures
“Skew-rank correlation”
Quadrant rank correlation (Smith, 2002)
etc, where I{A} is the indicator function of set A
3/233/133/133/232/122/12
22
S|'Y|E|'X|E|'Y|E|'X|Ea'YE'XE
'Y'XE'Y'XEa'Y'XEa;Y,Xρ
Y,XρY,XρY,XρY,XρY,Xρ SSSSS
22S
'VE'UE
,0'V,0'U'V'UEY,Xρ
I
22S'VE'UE
,0'V,0'U'V'UEY,Xρ
I