Winter School, Canazei. Frank A. Cowell. January 2010

Transcription

1 Winter School, Canazei Frank A. STICERD, London School of Economics January 2010

2 Meaning and motivation entropy uncertainty and entropy and inequality Entropy: "dynamic" aspects

3 Motivation What do we mean by? welfare analysis mainly about single-distribution comparisons examples: inequality, poverty, polarisation concerns a class of two-distributional problems Why two-distribution problems? effects of taxes and benefits? other economic applications involving reranking Is there a unifying approach? welfare economics? statistical tools? a general informational approach? Are standard tools appropriate?

4 This talk Informational analysis Background in information theory Information theory extensions Connections to income-distribution analysis measurement of goodness-of-fit measures and economics Axiomatisation Application indices goodness-of-fit criteria evaluation of some standard tools

5 Literature: Key references, F. A. (1985). of : An axiomatic approach. Review of Economic Studies 52, , F. A., E. Flachaire, and S. Bandyopadhyay (2009). Goodness-of-fit: An economic approach. Analysis Discussion Paper 101, STICERD, London School of Economics, London WC2A 2AE. Fields, G. S. and E. A. Ok (1999). Measuring movement of incomes. Economica 66, Van Kerm, P. (2004). What lies behind income? Reranking and in Belgium, Western Germany and the USA. Economica 71,

6 Modelling information Entropy is an aggregation of information about a system: the degree of disorder Consider a discrete set of events Θ := fθ 1,..., θ n g with probabilities fp 1,..., p n g An event θ i 2 Θ occurs model information content of this event as h : Θ! R What properties for the function h?

7 Valuing information The more unlikely is θ i the more valuable is the information that θ i occurred p i < p j implies h (p i ) > h p j For independent compound events value of information is additive h p i p j = h (pi ) + h p j Additive property is a log Cauchy equation solution h (p) = C log (p) where C is a constant Decreasingness property implies C is negative

8 Aggregating information Aggregate information by taking expectation Shannon entropy measure for discrete distribution: n i=1 p i log (p i ) max value is n i=1 n 1 log 1n = log (n) But why use this precise formulation?

9 Entropy: role of axioms Role of some axioms is clear decreasingness additivity of independent compound events But there is a hidden assumption why additivity in aggregation? implied by expectational approach Perhaps a more general approach would be worth while to generalise additivity of independent compound events to motivate additive aggregation

10 Entropy: a general approach Model entropy directly as H n : n! R and assume: 1 Continuity 2 Symmetry: H n (p 1, p 2, p 3,...) = H n (p 2, p 1, p 3,...) =... 3 Grouping axiom: 0 < m < n and p := m j=1 p i: H n (p 1,..., p n ) = ph m p1 p,..., p m p + [1 p] H n m pm+1 1 p,..., p n 1 p +H 2 (p, 1 p) 4 0-irrelevance: H n+1 (p 1,..., p n, 0) = H n (p 1,..., p n ) Then you get Shannon entropy

11 Entropy: extensions (1) Extension to continuous distributions probability density f over event space Θ information function h (f (θ)) where h is decreasing Entropy as an aggregation of information H(f ) := Eh (f (θ)) Shannon entropy measure: H(f ) = h (f ) = log f (θ) R log f (θ) f (θ)dθ Θ

12 Entropy: extensions (2) A number of information-theoretic arguments for a more general approach Focus on the nature of the function h rather than expectational aggregation Generalise h (f ) = log (f ) to become h (f ) = 1 α 1 1 f α 1 Get the α-class entropy H α (f ) = 1 α 1 1 E(f (θ) α 1 )

13 Entropy and inequality (1) Take x 2 X R + where x can be thought of as income If x has cdf F then income share function is s (q) := F 1 (q) R = x 1 0 F 1 (t)dt µ population normalised to 1 Use this to get entropy-based inequality measures apply entropy concept to s () rather than f () Theil inequality index I 1 := R x 0 µ log x µ df(x) where I 1 = H(s)

14 Entropy and inequality (2) Generalised entropy index I α = R hh i α x 0 µ 1i df(x) 1 α(α 1) where I α = α 1 H α (s) Important special cases Theil s second index I 0 = R 0 log xµ df(x) also known as MLD index R 0 [log (µ) log (x)] df(x) Atkinson indices Social values emerge implicitly choice of sensitivity index α α = 1 ɛ (for α < 1) gives Atkinson inequality aversion

15 Relative entropy (1) Use entropy to characterise changes in distributions relative entropy (equivalently) divergence entropy Divergence of f 2 with respect to f 1, is given by H(f 1, f 2 ) = R Θ h f1 f2 f 1 dθ relative entropy : expected information content in f 2 with respect to f 1 get standard entropy index as a special case Corresponding to α-class entropy get a class of divergence measures: H α (f 1, f 2 ) = α 1 R h i f1 α 1 1 Θ 1 f 1 f2 dθ

16 Relative entropy (2) Switch from probabilities to income shares: s 1 (q) = F 1 1 (q) R = x 1 0 F 1 1 (t)dt µ and s 2 (q) = R F 1 1 We can also apply relative entropy here: H 1 (s 1, s 2 ) = R 1 0 s 1 (q) log dq Raises a number of issues: how to interpret? generalisation? axiomatisation? s2 (q) s 1 (q) 1 2 (q) 0 F 1 2 (t)dt = y µ 2

17 Literature: Information theory Kullback, S. and R. A. Leibler (1951). On information and sufficiency. Annals of Mathematical Statistics 22, Havrda, J. and F. Charvat (1967). Quantification method in classification processes: concept of structural -entropy. Kybernetica 3, Renyi, G. (1961). On measures of entropy and information. In Proceedings of the Fourth Berkeley Symposium on Statistics, Volume 1: Probability, pp University of California Press. Shannon, C. E. (1948). A mathematical theory of communication. Bell System Technical Journal 106, and Theil, H. (1967). Economics and Information Theory, Chapter 4, pp Amsterdam: North Holland.

18 Dynamic aspect: change Revisit analogy between entropy (H) and inequality (I) Divergence entropy has a counterpart: measure of change measure: J α (x, y) := 1 nα(α 1) n i=1 h xi µ 1 i α h yi µ 2 i 1 α 1 J α (x, y) = α 1 H α (s 1, s 2 ) captures the average distance of an s 1 from a reference distribution s 2. J α (x, y) is an aggregate measure of discrepancy between two distributions use this to build analytical tools

19 Basics Representation of problem The axioms fundamental structure income scaling Characterisation theorems The index

20 : basics Purpose: to give meaning to the problem to avoid concealed arbitrariness Principles: parsimony: not impose too much mathematical structure consistency with axiomatisation of other economic problems Precedents: inequality welfare poverty

21 Representation of problem z : = (z 1, z 2,..., z n ) z i is the ordered pair (x i, y i ) Work with vector of discrepancies (D (z 1 ),..., D (z n )) discrepancy function D : Z! R D (z i ) strictly increasing in jx i y i j Two-step approach 1 characterise a weak ordering on Z n, the space of z 2 use the function representing to generate index J.

22 Basic axioms Continuity: is continuous on Z n Monotonicity: if z, z 0 2 Z n differ only in their ith component then D (x i, y i ) < D x 0 i, y0 i () z z 0 Symmetry: If z 0 is obtained by permuting the components of z: z s z 0. we can impose a simultaneous ordering on the x and y components of z Independence: If z s z 0 and z i = zi 0 for some i then z (ζ, i) s z 0 (ζ, i) for all ζ 2 [z i 1, z i+1 ] \ zi 0 1, z0 i+1 z (ζ, i) means replace ith component of z by ζ Perfect local fit: Suppose x i = y i, x j = y j, x 0 i = x i + δ, y 0 i = y i + δ, x 0 j = x j δ, y 0 j = y j δ and, for all k 6= i, j, x 0 k = x k, y 0 k = y k. Then z s z 0

23 First theorem Theorem: Given basic axioms is representable by n i=1 φ i (z i) 1 φ i is continuous and strictly decreasing in jx i y i j 2 φ i (x, x) = a i + b i x An additivity result We can evaluate focusing on one income-position at a time

24 Second theorem We need one more axiom Income scale irrelevance: If z s z 0 then tz s tz 0 for all t > 0 Theorem: Given conditions of first theorem and scale irrelevance is representable by φ n i=1 x xi ih i y i The function h i is essentially arbitrary we need to impose more structure do this in step 2

25 Income discrepancy and How to compare (x, y) discrepancies in different parts of the From Theorem 2 comparisons in terms of proportional differences: discrepancy should be D (z i ) = max xi y i, y i x i Discrepancy scale irrelevance: Suppose z 0 s z 0 0. Then z s z 0 for all z, z 0 such that D (z) = td (z 0 ) and D (z 0 ) = td (z 0 0 ) suppose vectors z 0 and z0 0 are equivalent under rescale all discrepancies in z 0 and z0 0 by the same factor t resulting pair of vectors z and z 0 will also be equivalent

26 The index Theorem: Given discrepancy scale irrelevance is representable by φ n i=1 xα i y1 α α 6= 1 is a constant Use the natural cardinalisation n i=1 xα i y1 i Normalise with reference to case where x i = µ 1 and y i = µ 2 for all i observed and modelled distribution exhibit complete equality h i α h i 1 This gives J α (x, y) := nα(α 1) n xi yi 1 α i=1 µ 1 µ 1 2 i α

27 Mobility concepts Mobility modelling Mobility measures Distance and : examples

28 Mobility concepts Variety of approaches ad hoc classification? focus on information theory? How does use information? use a pre-grouped scheme? use individual information in relation to others? use distance concept? Distance concept not only concerned with how many people move also we want to know how far some of this lost in the transition-matrix approach

29 Mobility modelling Basic information is the temporal pair z i = (x i,t 1, x i,t ) Bivariate distribution distribution function F (x t 1, x t ) marginal distributions F t 1 and F t give income distribution in each period Time-aggregated income derived from (x i,t 1, x i,t ) using weights w t 1, w t x i := w t 1 x i,t 1 + w t x i,t Distribution F derived from F

30 Mobility measures in practice Stability indices: 1 I(F w ) w t 1 I(F t 1 )+w t I(F t ) Hart (1976): 1 r(log x t 1, log x t ) where r is the correlation coefficient " R R x t e γr(f,(x t 1,x k t)) df(xt 1,x t ) King (1983): 1 µ k (F t ) k 1, k 6= 0, γ 0 where r(f; (x t 1, x t )) is a rank indicator: µ 1 (F t ) 1 jx t Q(F t ; F t 1 (x t 1 ))j Q(G; q) := inffx : G(x) qg Fields-Ok (1999): c R R jlog x t 1 log x t j df(x t 1, x t ) # 1 k

31 Mobility measures: distance M(F) := φ R D (x t 1, x t ) df (x t 1, x t ), µ(f) the function D : X X! R incorporates the distance concept If D is homothetic, the measure takes the form: Z Z h i 1 xt 1 α h i α 1 xt 1 df(x α 2 α µ(f t 1 ) µ(f t ) t 1, x t ) α is a sensitivity parameter

32 Mobility: example Van Kerm s comparison of in Europe and USA Uses trimmed panel data for each case Compares 1985, 1997 Belgium Germany USA Shorrocks (1978) Hart (1976) King (1983) Fields Ok (1999) Fields Ok (1996) Is the USA more mobile?

33 Literature: Mobility Fields, G. S. and E. A. Ok (1996). The meaning and measurement of income. Journal of Economic Theory 71, Fields, G. S. and E. A. Ok (1999). The measurement of income : An Introduction to the literature. In J. Silber (ed.), Handbook of Income Inequality Measurement. Dewenter: Kluwer. King, M. A. (1983). An index of inequality: with applications to horizontal equity and social. Econometrica 51, Shorrocks, A. F.. Income inequality and income. Journal of Economic Theory, 46, Van de gaer, D., E. Schokkaert, and M. Martinez (2001). Three meanings of intergenerational. Economica 68,

34 Goodness-of-Fit : general approach The EDF approach Quantile approach Aggregating discrepancy

35 EDF and Model problem requires representation of the facts and the model used to represent them x i : sample observations i = 1,..., n x i is a scalar Ēmpirical D istribution F unction ˆF (x) = 1 n n i=1 ι (x i x) ι (S) means statement S is true Modelled distribution F (x i ) could be continuous or discrete

36 The EDF approach What underlies the standard statistical approach Plot ˆF (x) and F (x) against x 1 ^ F * (x 3 ) F(x 3 ) F * ( ) x 3 x For each x i evaluate distance between F values

37 Literature: Goodness-of-Fit Anderson, T. W. and D. A. Darling (1952). Asymptotic theory of certain goodness of fit criteria based on stochastic processes. The Annals of Mathematical Statistics 23, Anderson, T. W. and D. A. Darling (1954). A test of goodness of fit. Journal of the American Statistical Association 49, d Agostino, R. B. and M. A. Stephens (1986). Goodness-of-Fit techniques. New York: Marcel Dekker. Stephens, M. A. (1974). EDF statistics for goodness of fit and some comparisons. Journal of the American Statistical Association 69,

38 Quantile approach A kind of dual approach Compute the quantiles F 1 (q) where q = i n+1 Transpose previous diagram: plot quantiles against q For each q evaluate distance between incomes on vertical axis

39 Aggregating discrepancy (1) Suppose distributions are discrete point masses one observes x proposed distribution is y = x+ x Consider three methods of evaluating overall discrepancy W(x) 1 Welfare loss: W (y) W (x) ' i x i x i if W is ordinal this is not a well-defined loss function also, can find x 6= 0 such that expression is zero I(x) 2 Inequality change: I (y) I (x) ' i x i x i same basic objections as for welfare loss 3 change: examine more closely!

40 Aggregating discrepancy (2) Consider a change in y: y 0 k = y k + δ, y 0 j = y j does the change y! y 0 move one closer to x? Ineq difference: up/down as y i? y j, irrespective of x change measure: up/down as y k y j x k x j y x j k δ? x k x j Use this to formalise (1) aggregate discrepancy, (2)

41 f0, f1, f Simulation f0 f1 f2 GE inequality f 0 f 1 f 2 I I f 0, f 1, f 2 each formed from a mixture of three lognormals f 1, f 0 similar in high incomes; f 2, f 0 similar in low incomes In inequality terms f 1 is closer than f 2 to the reference distribution f 0

42 Results for traditional criteria Simulate f 0, f 1 and f 2 using 10,000 simulated observations Compute Chi-squared criterion (χ 2 ) Also Cramér-von Mises (ω 2 ) f 1 f 2 χ ω f 2 is closer than f 1 to the reference distribution f 0!

43 Results for the J index Now compute J α 10 2 for a variety of α-values α f 1 f f 1 is closer than f 2 to the reference distribution f 0 (as with inequality) The higher is α, the closer is the approximation of f 1 to f 0 and the worse is that of f 2

44 Application to UK income data HBAI data. n: 3858, mean: , sd: Fit Singh-Maddala F SM (y; a, b, c) = 1 [1+ay b ] c MLE â, ˆb, ĉ = E 10, , ˆF (in red) and F SM (y; â, ˆb, ĉ)

45 results for traditional measures p (%) χ ω p values computed using bootstrap Singh-Maddala distribution is satisfactory (high p) Applies for both traditional criteria, χ 2 and ω 2

46 results for J α J α 10 2 p (%) α J α 10 2 p (%) J α criterion reveals a richer story p-values rise with α accept F SM as suitable for ˆF if J assigns higher weight to discrepancies at high incomes but for a bottom-sensitive criterion (α < 1) F SM regarded as unsatisfactory

47 What type of index? borrowed from stats? borrowed from inequality? distance approach Why do economists want to use criteria? evaluate suitability of a statistical models want a criterion based on economic principles J α indices form a class of criteria calibrate to suit the nature of the economic problem under consideration in which part of the distribution do you want the criterion to be sensitive? The choice of a fit criterion really matters off-the-shelf tools can be misleading J α answers accord with common sense α crucial to understanding whether model fits