Copula (probability theory)
In probability theory and statistics, a copula is a multivariate cumulative distribution function for which the marginal probability distribution of each variable is uniform. Copulas are used to describe the dependence between random variables. Their name comes from the Latin for "link" or "tie", similar but unrelated to grammatical copulas in linguistics. Copulas have been used widely in quantitative finance to model and minimize tail risk and portfolio-optimization applications.
Sklar's theorem states that any multivariate joint distribution can be written in terms of univariate marginal distribution functions and a copula which describes the dependence structure between the variables.
Copulas are popular in high-dimensional statistical applications as they allow one to easily model and estimate the distribution of random vectors by estimating marginals and copulae separately. There are many parametric copula families available, which usually have parameters that control the strength of dependence. Some popular parametric copula models are outlined below.
Two-dimensional copulas are known in some other areas of mathematics under the name permutons and doubly-stochastic measures.
- 1 Mathematical definition
- 2 Definition
- 3 Sklar's theorem
- 4 Stationary Condition
- 5 Fréchet–Hoeffding copula bounds
- 6 Families of copulas
- 7 Expectation for copula models and Monte Carlo integration
- 8 Empirical copulas
- 9 Applications
- 10 References
- 11 Further reading
- 12 External links
has uniformly distributed marginals.
The copula of is defined as the joint cumulative distribution function of :
The copula C contains all information on the dependence structure between the components of whereas the marginal cumulative distribution functions contain all information on the marginal distributions.
The importance of the above is that the reverse of these steps can be used to generate pseudo-random samples from general classes of multivariate probability distributions. That is, given a procedure to generate a sample from the copula distribution, the required sample can be constructed as
The inverses are unproblematic as the were assumed to be continuous. The above formula for the copula function can be rewritten to correspond to this as:
In analytic terms, is a d-dimensional copula if
- , the copula is zero if any one of the arguments is zero,
- , the copula is equal to u if one argument is u and all others 1,
- C is d-non-decreasing, i.e., for each hyperrectangle the C-volume of B is non-negative:
- where the .
For instance, in the bivariate case, is a bivariate copula if , and for all and .
of a random vector can be expressed in terms of its marginals and a copula . Indeed:
In case that the multivariate distribution has a density , and this is available, it holds further that
where is the density of the copula.
The theorem also states that, given , the copula is unique on , which is the cartesian product of the ranges of the marginal cdf's. This implies that the copula is unique if the marginals are continuous.
The converse is also true: given a copula and marginals then defines a d-dimensional cumulative distribution function.
(This section seems much less related to the main article)
When time series are auto-correlated, they may generate a non existence dependence between sets of variables and result in incorrect Copula dependence structure.
Fréchet–Hoeffding copula boundsEdit
The function W is called lower Fréchet–Hoeffding bound and is defined as
The function M is called upper Fréchet–Hoeffding bound and is defined as
The upper bound is sharp: M is always a copula, it corresponds to comonotone random variables.
The lower bound is point-wise sharp, in the sense that for fixed u, there is a copula such that . However, W is a copula only in two dimensions, in which case it corresponds to countermonotonic random variables.
In two dimensions, i.e. the bivariate case, the Fréchet–Hoeffding Theorem states
Families of copulasEdit
Several families of copulas have been described.
For a given correlation matrix , the Gaussian copula with parameter matrix can be written as
where is the inverse cumulative distribution function of a standard normal and is the joint cumulative distribution function of a multivariate normal distribution with mean vector zero and covariance matrix equal to the correlation matrix . While there is no simple analytical formula for the copula function, , it can be upper or lower bounded, and approximated using numerical integration. The density can be written as
where is the identity matrix.
Archimedean copulas are an associative class of copulas. Most common Archimedean copulas admit an explicit formula, something not possible for instance for the Gaussian copula. In practice, Archimedean copulas are popular because they allow modeling dependence in arbitrarily high dimensions with only one parameter, governing the strength of dependence.
A copula C is called Archimedean if it admits the representation
where is a continuous, strictly decreasing and convex function such that . is a parameter within some parameter space . is the so-called generator function and is its pseudo-inverse defined by
for all and and is nonincreasing and convex.
Most important Archimedean copulasEdit
The following tables highlight the most prominent bivariate Archimedean copulas, with their corresponding generator. [There is one problem with each of these two tables. In specific terms, and remain undefined in the first and second table, respectivey. They are neither defined in the main text of this page. Please define the variables.] Not all of them are completely monotone, i.e. d-monotone for all or d-monotone for certain only.
|Name of Copula||Bivariate Copula||parameter|
Expectation for copula models and Monte Carlo integrationEdit
In statistical applications, many problems can be formulated in the following way. One is interested in the expectation of a response function applied to some random vector . If we denote the cdf of this random vector with , the quantity of interest can thus be written as
If is given by a copula model, i.e.,
this expectation can be rewritten as
In case the copula C is absolutely continuous, i.e. C has a density c, this equation can be written as
and if each marginal distribution has the density it holds further that
If copula and margins are known (or if they have been estimated), this expectation can be approximated through the following Monte Carlo algorithm:
- Draw a sample of size n from the copula C
- By applying the inverse marginal cdf's, produce a sample of by setting
- Approximate by its empirical value:
When studying multivariate data, one might want to investigate the underlying copula. Suppose we have observations
from a random vector with continuous margins. The corresponding "true" copula observations would be
However, the marginal distribution functions are usually not known. Therefore, one can construct pseudo copula observations by using the empirical distribution functions
instead. Then, the pseudo copula observations are defined as
The corresponding empirical copula is then defined as
The components of the pseudo copula samples can also be written as , where is the rank of the observation :
Therefore, the empirical copula can be seen as the empirical distribution of the rank transformed data.
|Typical finance applications:
For the former, copulas are used to perform stress-tests and robustness checks that are especially important during "downside/crisis/panic regimes" where extreme downside events may occur (e.g., the global financial crisis of 2007–2008). The formula was also adapted for financial markets and was used to estimate the probability distribution of losses on pools of loans or bonds.
During a downside regime, a large number of investors who have held positions in riskier assets such as equities or real estate may seek refuge in 'safer' investments such as cash or bonds. This is also known as a flight-to-quality effect and investors tend to exit their positions in riskier assets in large numbers in a short period of time. As a result, during downside regimes, correlations across equities are greater on the downside as opposed to the upside and this may have disastrous effects on the economy. For example, anecdotally, we often read financial news headlines reporting the loss of hundreds of millions of dollars on the stock exchange in a single day; however, we rarely read reports of positive stock market gains of the same magnitude and in the same short time frame.
Copulas aid in analyzing the effects of downside regimes by allowing the modelling of the marginals and dependence structure of a multivariate probability model separately. For example, consider the stock exchange as a market consisting of a large number of traders each operating with his/her own strategies to maximize profits. The individualistic behaviour of each trader can be described by modelling the marginals. However, as all traders operate on the same exchange, each trader's actions have an interaction effect with other traders'. This interaction effect can be described by modelling the dependence structure. Therefore, copulas allow us to analyse the interaction effects which are of particular interest during downside regimes as investors tend to herd their trading behaviour and decisions. (See also agent-based computational economics, where price is treated as an emergent phenomenon, resulting from the interaction of the various market participants, or agents.)
The users of the formula have been criticized for creating "evaluation cultures" that continued to use simple copulæ despite the simple versions being acknowledged as inadequate for that purpose. Thus, previously, scalable copula models for large dimensions only allowed the modelling of elliptical dependence structures (i.e., Gaussian and Student-t copulas) that do not allow for correlation asymmetries where correlations differ on the upside or downside regimes. However, the recent development of vine copulas (also known as pair copulas) enables the flexible modelling of the dependence structure for portfolios of large dimensions. The Clayton canonical vine copula allows for the occurrence of extreme downside events and has been successfully applied in portfolio optimization and risk management applications. The model is able to reduce the effects of extreme downside correlations and produces improved statistical and economic performance compared to scalable elliptical dependence copulas such as the Gaussian and Student-t copula.
Other models developed for risk management applications are panic copulas that are glued with market estimates of the marginal distributions to analyze the effects of panic regimes on the portfolio profit and loss distribution. Panic copulas are created by Monte Carlo simulation, mixed with a re-weighting of the probability of each scenario.
As regards derivatives pricing, dependence modelling with copula functions is widely used in applications of financial risk assessment and actuarial analysis – for example in the pricing of collateralized debt obligations (CDOs). Some believe the methodology of applying the Gaussian copula to credit derivatives to be one of the reasons behind the global financial crisis of 2008–2009; see David X. Li § CDOs and Gaussian copula.
Despite this perception, there are documented attempts within the financial industry, occurring before the crisis, to address the limitations of the Gaussian copula and of copula functions more generally, specifically the lack of dependence dynamics. The Gaussian copula is lacking as it only allows for an elliptical dependence structure, as dependence is only modeled using the variance-covariance matrix. This methodology is limited such that it does not allow for dependence to evolve as the financial markets exhibit asymmetric dependence, whereby correlations across assets significantly increase during downturns compared to upturns. Therefore, modeling approaches using the Gaussian copula exhibit a poor representation of extreme events. There have been attempts to propose models rectifying some of the copula limitations.
Additional to CDOs, Copulas have been applied to other asset classes as a flexible tool in analyzing multi-asset derivative products. The first such application outside credit was to use a copula to construct a basket implied volatility surface, taking into account the volatility smile of basket components. Copulas have since gained popularity in pricing and risk management of options on multi-assets in the presence of a volatility smile, in equity-, foreign exchange- and fixed income derivatives.
Recently, copula functions have been successfully applied to the database formulation for the reliability analysis of highway bridges, and to various multivariate simulation studies in civil engineering, reliability of wind and earthquake engineering, and mechanical & offshore engineering. Researchers are also trying these functions in the field of transportation to understand the interaction between behaviors of individual drivers which, in totality, shapes traffic flow.
Warranty data analysisEdit
The combination of SSA and Copula-based methods have been applied for the first time as a novel stochastic tool for polar motion prediction. 
Climate and weather researchEdit
Solar irradiance variabilityEdit
Random vector generationEdit
Large synthetic traces of vectors and stationary time series can be generated using empirical copula while preserving the entire dependence structure of small datasets. Such empirical traces are useful in various simulation-based performance studies.
- Low, R.K.Y.; Alcock, J.; Faff, R.; Brailsford, T. (2013). "Canonical vine copulas in the context of modern portfolio management: Are they worth it?". Journal of Banking & Finance. 37 (8): 3085–3099. doi:10.1016/j.jbankfin.2013.02.036.
- Low, R.K.Y.; Faff, R.; Aas, K. (2016). "Enhancing mean–variance portfolio selection by modeling distributional asymmetries" (PDF). Journal of Economics and Business. 85: 49–72. doi:10.1016/j.jeconbus.2016.01.003.
- Nelsen, Roger B. (1999), An Introduction to Copulas, New York: Springer, ISBN 978-0-387-98623-4
- Sklar, A. (1959), "Fonctions de répartition à n dimensions et leurs marges", Publ. Inst. Statist. Univ. Paris, 8: 229–231
- Durante, Fabrizio; Fernández-Sánchez, Juan; Sempi, Carlo (2013), "A Topological Proof of Sklar's Theorem", Applied Mathematics Letters, 26 (9): 945–948, doi:10.1016/j.aml.2013.04.005
- Sadegh, Mojtaba; Ragno, Elisa; AghaKouchak, Amir (2017). "Multivariate Copula Analysis Toolbox (MvCAT): Describing dependence and underlying uncertainty using a Bayesian framework". Water Resources Research. 53 (6): 5166–5183. Bibcode:2017WRR....53.5166S. doi:10.1002/2016WR020242. ISSN 1944-7973.
- AghaKouchak, Amir; Bárdossy, András; Habib, Emad (2010). "Copula-based uncertainty modelling: application to multisensor precipitation estimates". Hydrological Processes. 24 (15): 2111–2124. doi:10.1002/hyp.7632. ISSN 1099-1085.
- J. J. O'Connor and E. F. Robertson (March 2011). "Biography of Wassily Hoeffding". School of Mathematics and Statistics, University of St Andrews, Scotland. Retrieved 14 February 2019.
- Botev, Z. I. (2016). "The normal law under linear restrictions: simulation and estimation via minimax tilting". Journal of the Royal Statistical Society, Series B. 79: 125–148. arXiv:1603.04166. Bibcode:2016arXiv160304166B. doi:10.1111/rssb.12162.
- Botev, Zdravko I. (10 November 2015). "TruncatedNormal: Truncated Multivariate Normal" – via R-Packages.
- Arbenz, Philipp (2013). "Bayesian Copulae Distributions, with Application to Operational Risk Management—Some Comments". Methodology and Computing in Applied Probability. 15 (1): 105–108. doi:10.1007/s11009-011-9224-0.
- Nelsen, R. B. (2006). An Introduction to Copulas (Second ed.). New York: Springer. ISBN 978-1-4419-2109-3.
- McNeil, A. J.; Nešlehová, J. (2009). "Multivariate Archimedean copulas, d-monotone functions and 1-norm symmetric distributions". Annals of Statistics. 37 (5b): 3059–3097. arXiv:0908.3750. doi:10.1214/07-AOS556.
- Ali, M. M.; Mikhail, N. N.; Haq, M. S. (1978), "A class of bivariate distributions including the bivariate logistic", J. Multivariate Anal., 8 (3): 405–412, doi:10.1016/0047-259X(78)90063-5
- Clayton, David G. (1978). "A model for association in bivariate life tables and its application in epidemiological studies of familial tendency in chronic disease incidence". Biometrika. 65 (1): 141–151. doi:10.1093/biomet/65.1.141. JSTOR 2335289.
- Alexander J. McNeil, Rudiger Frey and Paul Embrechts (2005) "Quantitative Risk Management: Concepts, Techniques, and Tools", Princeton Series in Finance
- Low, Rand (2017-05-11). "Vine copulas: modelling systemic risk and enhancing higher-moment portfolio optimisation". Accounting & Finance. 58: 423–463. doi:10.1111/acfi.12274.
- Rad, Hossein; Low, Rand Kwong Yew; Faff, Robert (2016-04-27). "The profitability of pairs trading strategies: distance, cointegration and copula methods". Quantitative Finance. 16 (10): 1541–1558. doi:10.1080/14697688.2016.1164337.
- Longin, F; Solnik, B (2001), "Extreme correlation of international equity markets", Journal of Finance, 56 (2): 649–676, CiteSeerX 10.1.1.321.4899, doi:10.1111/0022-1082.00340
- Ang, A; Chen, J (2002), "Asymmetric correlations of equity portfolios", Journal of Financial Economics, 63 (3): 443–494, doi:10.1016/s0304-405x(02)00068-5
- MacKenzie, Donald; Spears, Taylor (June 2012). "The Formula That Killed Wall Street"? The Gaussian Copula and the Material Cultures of Modelling (pdf) (Technical report). University of Edinburgh School of Social and Political Sciences.
- Cooke, R.M.; Joe, H.; Aas, K. (January 2011). Kurowicka, D.; Joe, H. (eds.). Dependence Modeling Vine Copula Handbook (PDF). World Scientific. pp. 37–72. ISBN 978-981-4299-87-9.
- Aas, K; Czado, C; Bakken, H (2009), "Pair-copula constructions of multiple dependence", Insurance: Mathematics and Economics, 44 (2): 182–198, CiteSeerX 10.1.1.61.3984, doi:10.1016/j.insmatheco.2007.02.001
- Low, R; Alcock, J; Brailsford, T; Faff, R (2013), "Canonical vine copulas in the context of modern portfolio management: Are they worth it?", Journal of Banking and Finance, 37 (8): 3085–3099, doi:10.1016/j.jbankfin.2013.02.036
- Meucci, Attilio (2011), "A New Breed of Copulas for Risk and Portfolio Management", Risk, 24 (9): 122–126
- Meneguzzo, David; Vecchiato, Walter (Nov 2003), "Copula sensitivity in collateralized debt obligations and basket default swaps", Journal of Futures Markets, 24 (1): 37–70, doi:10.1002/fut.10110
- Recipe for Disaster: The Formula That Killed Wall Street Wired, 2/23/2009
- MacKenzie, Donald (2008), "End-of-the-World Trade", London Review of Books (published 2008-05-08), pp. 24–26, retrieved 2009-07-27
- Jones, Sam (April 24, 2009), "The formula that felled Wall St", Financial Times
- Lipton, Alexander; Rennie, Andrew (2008). Credit Correlation: Life After Copulas. World Scientific. ISBN 978-981-270-949-3.
- Donnelly, C; Embrechts, P (2010). "The devil is in the tails: actuarial mathematics and the subprime mortgage crisis". ASTIN Bulletin 40(1), 1–33. Cite journal requires
- Brigo, D; Pallavicini, A; Torresetti, R (2010). Credit Models and the Crisis: A Journey into CDOs, Copulas, Correlations and dynamic Models. Wiley and Sons.
- Qu, Dong (2001). "Basket Implied Volatility Surface". Derivatives Week (4 June).
- Qu, Dong (2005). "Pricing Basket Options With Skew". Wilmott Magazine (July).
- Thompson, David; Kilgore, Roger (2011), "Estimating Joint Flow Probabilities at Stream Confluences using Copulas", Transportation Research Record, 2262: 200–206, doi:10.3141/2262-20, retrieved 2012-02-21
- Yang, S.C.; Liu, T.J.; Hong, H.P. (2017). "Reliability of Tower and Tower-Line Systems under Spatiotemporally Varying Wind or Earthquake Loads". Journal of Structural Engineering. 143 (10): 04017137. doi:10.1061/(ASCE)ST.1943-541X.0001835.
- Zhang, Yi; Beer, Michael; Quek, Ser Tong (2015-07-01). "Long-term performance assessment and design of offshore structures". Computers & Structures. 154: 101–115. doi:10.1016/j.compstruc.2015.02.029.
- Pham, Hong (2003), Handbook of Reliability Engineering, Springer, pp. 150–151
- Wu, S. (2014), "Construction of asymmetric copulas and its application in two-dimensional reliability modelling" (PDF), European Journal of Operational Research, 238 (2): 476–485, doi:10.1016/j.ejor.2014.03.016
- Ruan, S.; Swaminathan, N; Darbyshire, O (2014), "Modelling of turbulent lifted jet flames using flamelets: a priori assessment and a posteriori validation", Combustion Theory and Modelling, 18 (2): 295–329, Bibcode:2014CTM....18..295R, doi:10.1080/13647830.2014.898409
- Darbyshire, O.R.; Swaminathan, N (2012), "A presumed joint pdf model for turbulent combustion with varying equivalence ratio", Combustion Science and Technology, 184 (12): 2036–2067, doi:10.1080/00102202.2012.696566
- Eban, E; Rothschild, R; Mizrahi, A; Nelken, I; Elidan, G (2013), Carvalho, C; Ravikumar, P (eds.), "Dynamic Copula Networks for Modeling Real-valued Time Series" (PDF), Journal of Machine Learning Research, 31
- Onken, A; Grünewälder, S; Munk, MH; Obermayer, K (2009), Aertsen, Ad (ed.), "Analyzing Short-Term Noise Dependencies of Spike-Counts in Macaque Prefrontal Cortex Using Copulas and the Flashlight Transformation", PLoS Computational Biology, 5 (11): e1000577, Bibcode:2009PLSCB...5E0577O, doi:10.1371/journal.pcbi.1000577, PMC 2776173, PMID 19956759
- Modiri, S.; Belda, S.; Heinkelmann, R.; Hoseini, M.; Ferrándiz, J.M.; Schuh, H. (2018). "Polar motion prediction using the combination of SSA and Copula-based analysis". Earth, Planets and Space. 70 (70): 115. Bibcode:2018EP&S...70..115M. doi:10.1186/s40623-018-0888-3. PMC 6434970. PMID 30996648.
- Laux, P.; Wagner, S.; Wagner, A.; Jacobeit, J.; Bárdossy, A.; Kunstmann, H. (2009). "Modelling daily precipitation features in the Volta Basin of West Africa". Int. J. Climatol. 29 (7): 937–954. Bibcode:2009IJCli..29..937L. doi:10.1002/joc.1852.
- Schölzel, C.; Friederichs, P. (2008). "Multivariate non-normally distributed random variables in climate research – introduction to the copula approach". Nonlinear Processes in Geophysics. 15 (5): 761–772. doi:10.5194/npg-15-761-2008.
- Laux, P.; Vogl, S.; Qiu, W.; Knoche, H.R.; Kunstmann, H. (2011). "Copula-based statistical refinement of precipitation in RCM simulations over complex terrain". Hydrol. Earth Syst. Sci. 15 (7): 2401–2419. Bibcode:2011HESS...15.2401L. doi:10.5194/hess-15-2401-2011.
- Munkhammar, J.; Widén, J. (2017). "A copula method for simulating correlated instantaneous solar irradiance in spatial networks". Solar Energy. 143: 10–21. Bibcode:2017SoEn..143...10M. doi:10.1016/j.solener.2016.12.022.
- Munkhammar, J.; Widén, J. (2017). "An autocorrelation-based copula model for generating realistic clear-sky index time-series". Solar Energy. 158: 9–19. Bibcode:2017SoEn..158....9M. doi:10.1016/j.solener.2017.09.028.
- Strelen, Johann Christoph (2009). Tools for Dependent Simulation Input with Copulas. 2nd International ICST Conference on Simulation Tools and Techniques. doi:10.4108/icst.simutools2009.5596.
- Bandara, H. M. N. D.; Jayasumana, A. P. (Dec 2011). On Characteristics and Modeling of P2P Resources with Correlated Static and Dynamic Attributes. IEEE Globecom. pp. 1–6. CiteSeerX 10.1.1.309.3975. doi:10.1109/GLOCOM.2011.6134288. ISBN 978-1-4244-9268-8.
- The standard reference for an introduction to copulas. Covers all fundamental aspects, summarizes the most popular copula classes, and provides proofs for the important theorems related to copulas
- A book covering current topics in mathematical research on copulas:
- A reference for sampling applications and stochastic models related to copulas is
- A paper covering the historic development of copula theory, by the person associated with the "invention" of copulas, Abe Sklar.
- The standard reference for multivariate models and copula theory in the context of financial and insurance models
- Hazewinkel, Michiel, ed. (2001) , "Copula", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4
- Copula Wiki: community portal for researchers with interest in copulas
- A collection of Copula simulation and estimation codes
- Thorsten Schmidt (2006) "Coping with Copulas"
- Copulas & Correlation using Excel Simulation Articles
- Chapter 1 of Jan-Frederik Mai, Matthias Scherer (2012) "Simulating Copulas: Stochastic Models, Sampling Algorithms, and Applications"