# Using the Delta Method to Construct Confidence Intervals for Predicted Probabilities, Rates, and Discrete Changes

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1 Using the Delta Method to Construct Confidence Intervals for Predicted Probabilities, Rates, Discrete Changes JunXuJ.ScottLong Indiana University August 22, 2005 The paper provides technical details on the methods described in Jun Xu J. Scott Long (forthcoming) Confidence Intervals for Predicted Outcomes in Regression Models for Categorical Outcomes The Stata Journal. These formula were incorporated into prvalue. See for further details. 1 General Formula The delta method is a general approach for computing confidence intervals for functions of maximum likelihood estimates. The delta method takes a function that is too complex for analytically computing the variance, creates a linear approximation of that function, then computes the variance of the simpler linear function that can be used for large sample inference. We begin with a general result for maximum likelihood theory. Under stard regularity conditions, if β b is a vector of ML estimates, then ³ h ³ n bβ β d N 0,V ar bβ i. (1) Let G (β) be some function, such as predicted probabilities from a logit or ordinal logit model. The Taylor series expansion of G( b β) is G( β) b G(β)+( β b β) 0 G 0 (β)+( β b β) 0 G 00 (β )( β b β)/2 (2) G(β)+( β b β) 0 G 0 (β), where G 0 (β) G 00 (β) are matrices of first second partial derivatives with respect to β, β is some value between β b β. Then, h n G( β) b i G(β) n( β b β) 0 G 0 (β). (3) This leads to leads to G( β) b ³ N G(β), G(β) Var( b β) G(β) (Greene 2000; Agresti 2002). 0 To estimate the variance, we evaluate the partials at the ML estimates, G(β x),which β 0 β leads to ³ Var G( β) b G(b β) b 0 Var( β) b G(b β) b. (4) 1

2 Forexample,considerthelogitmodelwith G(β) Pr(y 1 x) Λ (x 0 β). (5) To compute the confidence interval, we need the gradient vector G(β) h Λ(x0 β) 0 Λ(x 0 β) 1 Λ(x 0 β) K i 0. (6) Since Λ is a cdf, Λ(x0 β) Λ(x0 β) k x 0 β x 0 β k λ (x 0 β) x k.then G(β) λ (x 0 β) λ (x 0 β) x 1 λ (x 0 β) x K 0. (7) To compute the confidence interval for a change in the probability as the independent variables change from x a to x b, we use the function G (β) Λ(β x a ) Λ (β x b ), (8) where G(β) [Λ(β x a) Λ(β x b )] Λ(β x a) Λ(β x b). (9) Substituting this result into equation 4, Λ(β xa ) Var(G(β)) 0 Var( β) b Λ(β x a) Λ(β xa ) 0 Var( β) b Λ(β x b) Λ(β xb ) 0 Var( β) b Λ(β x a) Λ(β xb ) + 0 Var( β) b Λ(β x b) (10). We now apply these formula to the models for which prvalue computes confidence intervals. 2 Binary Models In binary models, G(β) Pr(y 1 x) F (x 0 β) where F is the cdf for the logistic, normal, or cloglog function. The gradient is F(x 0 β) F(x0 β) k x 0 β x 0 β k f(x 0 β)x k, (11) where f is the pdf corresponding to F.Forthevectorx it follows that F(x 0 β) f(x 0 β)x. (12) 2

3 From equation 4, Var [Pr(y 1 x)] f(x 0 β)x 0 Var( β)xf(x b 0 β) f(x 0 β) 2 x 0 Var( β)x b. (13) The variances of Pr(y 0 x) Pr(y 0 x) are the equal since [1 F (x 0 β)] f(x 0 β)x (14) Var [Pr(y 0 x)] [ f(x 0 β)] 2 x 0 Var( b β)x. (15) 3 Ordered Logit Probit Assume that there are m 1,J outcome categories, where Pr (y m x) F (τ m x 0 β) F (τ m 1 x 0 β) for j 1,J. (16) Since we assume that τ 0 τ J, F (τ 0 x 0 β)0 F (τ J x 0 β)1. To compute the gradient, It follows that F (τ m x 0 β) F (τ m x 0 β) (τ m x 0 β) (17) k (τ m x 0 β) k f (τ m x 0 β)( x k ) (18) F (τ m x 0 β) F (τ m x 0 β) τ j (τ m x 0 β) (τ m x 0 β) τ j. (19) F (τ m x 0 β) τ j f (τ m x 0 β) if j m (20) F (τ m x 0 β) 0if j 6 m. (21) τ j Using these results with equation 16, Pr (y i m x i ) k [f (τ m x 0 β)( x k )] [f (τ m 1 x 0 β)( x k )] (22) x k f (τ m x 0 β) [f (τ m 1 x 0 β)] Pr (y i m x i ) F (τ m x 0 β) F (τ m 1 x 0 β) (23) τ j τ j τ j f (τ m x 0 β) if j m f (τ m 1 x 0 β) if j m 1 0 otherwise. 3

4 then For example, with three categories: With respect to τ, Pr (y 1 x) F (τ 1 x 0 β) 0 (24) Pr (y 2 x) F (τ 2 x 0 β) F (τ 1 x 0 β) (25) Pr (y 3 x) 1 F (τ 2 x 0 β), (26) Pr (y i 1 x i ) k x k [f (τ 1 x 0 β)] (27) Pr (y i 2 x i ) k x k [f (τ 2 x 0 β) f (τ 1 x 0 β)] (28) Pr (y i 3 x i ) k x k [ f (τ 2 x 0 β)]. (29) Pr (y i 1 x i ) τ 1 f (τ 1 x 0 β) (30) Pr (y i 1 x i ) τ 2 0 (31) Pr (y i 2 x i ) τ 1 f (τ 1 x 0 β) (32) Pr (y i 2 x i ) τ 2 f (τ 2 x 0 β) (33) Pr (y i 3 x i ) τ 1 0 (34) Pr (y i 3 x i ) τ 2 0. (35) To implement these procedures in Stata, we create the augmented matrices: β β 0 τ 1 τ J 1 0 (36) x 1 x x 2 x (37). x J 1 x , such that x 0 j β τ j xβ. (38) We then create the gradients described above. 4

5 4 Generalized Ordered Logit 1 The generalized ordered logit model is identical to the ordinal logit model except that the coefficients associated with x differ for each outcome. Since there is an intercept for each outcome, the τ s are fixed to zero β J 0 for identification. Then, Pr (y i 1 x i ) F ( x 0 β m ) (39) Pr (y i m x i ) F ( x 0 β m ) F x 0 β m 1 for m 2,J 1 (40) Pr (y i J x i ) F x 0 β m 1. (41) The gradient with respect to the β s is while no gradient for thresholds is needed. Then, 5 Multinomial Logit Assuming outcomes 1 through J, F ( x 0 β m ) m,k f ( x 0 β m )( x k ), (42) Pr (y i m x i ) m,k x k f ( x 0 β m ) f x 0 β m 1. (43) Pr(y m x) exp (xβ m) JP exp, (44) xβ j where without loss of generality we assume that β 1 0 to identify the model ( accordingly, the derivatives below do not apply to the partial with respect to β 1 ). To simplify notation, let P exp xβ j. The derivative of the probability of m with respect to βn is Using the quotient rule, Pr(y m x) n Examining each partial in turn. Pr(y m x) n j1 exp (xβ m) 1 n. (45) exp (xβ m) exp (xβ m ) 2. (46) n n exp (xβ m ) exp (xβ m) m (47) n m n exp(xβ m ) x if m n 0 if m 6 n 1 Confidence intervals for the generalized ordered logit model are not supported in prvalue. 5

6 P J exp xβ j JP exp xβ j j1 j1 n n (48) exp(xβ n ) x. The last equality follows since the partial of exp xβ j with respect to βn is 0 unless j n. Combining these results. If m n, Pr(y m x) exp (xβ m ) x exp (xβ m ) 2 x 2 (49) m exp (xβ m ) exp (xβ m ) 2 2 x exp (xβm ) exp (xβ m) exp (xβ m ) x [Pr(y m) Pr (y m)pr(y m)] x Pr(y m)[1 Pr (y m)] x. For m 6 n, Pr(y m x) [0 exp (xβ m )exp(xβ n ) x] 2 (50) n exp (xβ m) exp (xβ n ) x Pr(y m)pr(y n) x. For example, for two x s m 1: Pr(y m x) p m (1 p m ) x 1 p m (1 p m ) x 2 p m (1 p m ) 0 (51) m Pr(y m x) 0 p m p n x 1 p m p n x 2 p m p n. (52) n6m 6 Poisson Negative Binomial Regression In the Poisson regression model, so that μ i exp(x 0 iβ), (53) μ exp (x0 β) (54) k k exp(x0 β) x 0 β x 0 β k exp(x 0 β)x k μx k 6

7 Using matrices, μ exp (x0 β) exp(x0 β) x 0 β x 0 β μx. (55) The probability of a given count is so we can compute the gradient as: Pr (y x) exp ( μ) μy y!, (56) exp ( μ) μ y /y! 1 exp ( μ) μ y μ. (57) k y! μ k Since the last term was computed above, we only need to derive exp ( μ) μ y μ exp( μ) μy exp ( μ) + μy μ μ exp( μ) yμ y 1 μ y exp ( μ). (58) This leads to Using matrices, Pr (y x) 1 k y! μ exp ( μ) yμ y 1 μ y exp ( μ) x k (59) exp ( μ) yμy μ y+1 exp ( μ) x k y! yμy μ y+1 exp (μ) y! x k. Pr (y x) The negative binomial model is specified as exp ( μ) yμy μ y+1 exp ( μ) x (60) y! yμy μ y+1 exp (μ) y! x. μ exp(x 0 β + ε) (61) exp(x 0 β)exp(ε), where ε has a gamma distribution with variance α. The counts have a negative binomial distribution Pr (y i x i ) Γ(y µ ν µ yi i + ν) ν μi, (62) y i! Γ(ν) ν + μ i ν + μ i 7

8 where ν α 1. The derivatives of the log-likelihood are given in Stata Reference, Version 8, page 10. To simplify notation, we define τ lnα, m 1/α, p 1/ (1 + αμ), μ exp(xβ). Withψ (z) being the digamma function evaluated at z, τ p (y μ) (63) α (μ y) m ln (1 + αμ)+ψ (y + m) ψ (m). 1+αμ (64) Then by the chain rule, Pr (y x) Pr (y x) Pr(y x) 1 Pr (y x), (65) so that Similarly for τ, Pr (y x) Pr (y x) τ τ Pr (y x). (66) Pr (y x). (67) 7 References Agresti, Alan Categorical Data Analysis. 2nd Edition. New York: Wiley. Greene, William H Econometric Analysis, 4th Ed. New York: Prentice Hall. File: spost_deltaci-22aug2005.tex - 22Aug2005 8

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